lib: extract rt module from os, sweep imports

Hare puts runtime allocation in rt::, not os:: (ref/hare/rt/malloc.ha:27,
README). ww's `@symbol("rt_alloc") fn alloc(n: u64) *void;` lived at
lib/os/os.ww as a historical bootstrap shortcut; this commit relocates
it to a new lib/rt/malloc.ww and sweeps every site that depended on
`import os` for the alloc decl over to `import rt`.

This is commit 1 of 3 in the lib/rt extraction (#35):
  1. (this) move decl, sweep imports — preserves shape
  2. rename rt_alloc → rt_malloc (#38)
  3. nullable return type + OOM-propagating builtin lowering (#39)

No rename here. Symbol stays rt_alloc, function stays `alloc`, return
stays *void. Behavior identical — same ffi resolution outcome, just
sourced from a different module file. The rt::ensure runtime helper at
selfhost/rt/ensure.ww is its own compilation unit with a local decl and
is untouched.

Side effect: every wcc cgen file used `rt` as a local *node variable
name for "return type." `import rt;` shadows the module, so each
selfhost/cmd/wcc/{check,cgenstmt,cgenexpr,cgenutil}.ww site renamed
to `rtyp`. Mechanical follow-through; only the wcc module-import was
forced to do this rename.

Verified 132/132 + 995_self_rebuild byte-identity (5 wwstage tools
round-trip byte-identical).

selfhost/cmd/wcc/cgenexpr.ww

@@ -171,10 +171,10 @@ fn cgtryprop(c: *cgen, n: *node) void = {
 					};
 				};
 				if (cname.len > 0) {
-					let rt: *node = fnretlookupmod(c, cname, cmod);
-					if (rt != nil) {
-						if (rt.kind == nkind.N_TTAGGED) {
-							let first: *node = rt.list;
+					let rtyp: *node = fnretlookupmod(c, cname, cmod);
+					if (rtyp != nil) {
+						if (rtyp.kind == nkind.N_TTAGGED) {
+							let first: *node = rtyp.list;
 							if (first != nil) {
 								if (isstrtype(c, first)) {
 									succisstr = true;
@@ -227,10 +227,10 @@ fn cgtryunw(c: *cgen, n: *node) void = {
 					};
 				};
 				if (cname.len > 0) {
-					let rt: *node = fnretlookupmod(c, cname, cmod);
-					if (rt != nil) {
-						if (rt.kind == nkind.N_TTAGGED) {
-							let first: *node = rt.list;
+					let rtyp: *node = fnretlookupmod(c, cname, cmod);
+					if (rtyp != nil) {
+						if (rtyp.kind == nkind.N_TTAGGED) {
+							let first: *node = rtyp.list;
 							if (first != nil) {
 								if (isstrtype(c, first)) {
 									succisstr = true;
@@ -607,8 +607,8 @@ fn cgident(c: *cgen, n: *node) void = {
 	// in one go, so a body-less FFI binding emits the C symbol
 	// it was declared with via @symbol(), not the ww-side ident.
 	// Bare ident → same-module by ww's resolver, hint with c.curmod.
-	let rt: *node = fnretlookup(c, nm);
-	if (rt != nil) {
+	let rtyp: *node = fnretlookup(c, nm);
+	if (rtyp != nil) {
 		emitline("\tLEAQ\t");
 		emitfnname(c, nm, c.curmod);
 		emitline("(SB), AX\n");
@@ -3421,8 +3421,8 @@ fn cgcall(c: *cgen, n: *node) void = {
 				};
 			};
 		};
-		let rt: *node = fnretlookupmod(c, calleename, cmod);
-		if (isstrtype(c, rt)) {
+		let rtyp: *node = fnretlookupmod(c, calleename, cmod);
+		if (isstrtype(c, rtyp)) {
 			emitline("\tMOVQ\tDX, BX\n");
 		};
 	};

selfhost/cmd/wcc/cgenstmt.ww

@@ -184,8 +184,8 @@ fn cgreturn(c: *cgen, n: *node) void = {
 						};
 					};
 					if (calleename.len > 0) {
-						let rt: *node = fnretlookupmod(c, calleename, cmod);
-						if (istaggedtype(c, rt)) { forwardtagged = true; };
+						let rtyp: *node = fnretlookupmod(c, calleename, cmod);
+						if (istaggedtype(c, rtyp)) { forwardtagged = true; };
 					};
 				};
 			};
@@ -1225,10 +1225,10 @@ fn cgmlet(c: *cgen, n: *node) void = {
 				};
 			};
 			if (cnm.len > 0) {
-				let rt: *node = fnretlookupmod(c, cnm, cmod);
-				if (rt != nil) {
-					if (rt.kind == nkind.N_TTUPLE) {
-						p0t = rt.list;
+				let rtyp: *node = fnretlookupmod(c, cnm, cmod);
+				if (rtyp != nil) {
+					if (rtyp.kind == nkind.N_TTUPLE) {
+						p0t = rtyp.list;
 						if (p0t != nil) { p1t = p0t.next; };
 					};
 				};

selfhost/cmd/wcc/cgenutil.ww

@@ -516,9 +516,9 @@ export fn taggedcallslot(c: *cgen, n: *node) i32 = {
 	let callee: *node = n.lhs;
 	if (callee == nil) { return 0; };
 	if (callee.kind != nkind.N_IDENT) { return 0; };
-	let rt: *node = fnretlookup(c, callee.str);
-	if (!istaggedtype(c, rt)) { return 0; };
-	return slotsize(c, rt);
+	let rtyp: *node = fnretlookup(c, callee.str);
+	if (!istaggedtype(c, rtyp)) { return 0; };
+	return slotsize(c, rtyp);
 };
 
 fn nodeisslice(c: *cgen, n: *node) bool = {
@@ -547,8 +547,8 @@ fn nodeisslice(c: *cgen, n: *node) bool = {
 		let callee: *node = n.lhs;
 		if (callee != nil) {
 			if (callee.kind == nkind.N_IDENT) {
-				let rt: *node = fnretlookupmod(c, callee.str, c.curmod);
-				return isslicetype(c, rt);
+				let rtyp: *node = fnretlookupmod(c, callee.str, c.curmod);
+				return isslicetype(c, rtyp);
 			};
 			if (callee.kind == nkind.N_DOT) {
 				let cmod: str;
@@ -558,8 +558,8 @@ fn nodeisslice(c: *cgen, n: *node) bool = {
 						cmod = callee.lhs.str;
 					};
 				};
-				let rt: *node = fnretlookupmod(c, callee.str, cmod);
-				return isslicetype(c, rt);
+				let rtyp: *node = fnretlookupmod(c, callee.str, cmod);
+				return isslicetype(c, rtyp);
 			};
 		};
 		return false;
@@ -674,8 +674,8 @@ fn nodeisstr(c: *cgen, n: *node) bool = {
 		let callee: *node = n.lhs;
 		if (callee != nil) {
 			if (callee.kind == nkind.N_IDENT) {
-				let rt: *node = fnretlookupmod(c, callee.str, c.curmod);
-				return isstrtype(c, rt);
+				let rtyp: *node = fnretlookupmod(c, callee.str, c.curmod);
+				return isstrtype(c, rtyp);
 			};
 			// #34: cross-module N_DOT — route through fnretlookupmod
 			// so a same-leaf caller-module fn (different return shape)
@@ -688,8 +688,8 @@ fn nodeisstr(c: *cgen, n: *node) bool = {
 						cmod = callee.lhs.str;
 					};
 				};
-				let rt: *node = fnretlookupmod(c, callee.str, cmod);
-				return isstrtype(c, rt);
+				let rtyp: *node = fnretlookupmod(c, callee.str, cmod);
+				return isstrtype(c, rtyp);
 			};
 		};
 		return false;
@@ -1605,8 +1605,8 @@ export fn callsretsize(c: *cgen, n: *node) i32 = {
 		};
 	};
 	if (cn.len == 0) { return 0; };
-	let rt: *node = fnretlookupmod(c, cn, cmod);
-	return sretretsize(c, rt);
+	let rtyp: *node = fnretlookupmod(c, cn, cmod);
+	return sretretsize(c, rtyp);
 };
 
 fn structlookup(c: *cgen, name: str) *structinfo = {
@@ -1863,16 +1863,16 @@ fn inferletcalltype(c: *cgen, rhs: *node) *node = {
 		};
 	};
 	if (cname.len == 0) { return nil; };
-	let rt: *node = fnretlookupmod(c, cname, cmod);
-	if (rt == nil) { return nil; };
+	let rtyp: *node = fnretlookupmod(c, cname, cmod);
+	if (rtyp == nil) { return nil; };
 	if (unwrap) {
 		// Strip error variants — success type is the first
 		// variant of the tagged return.
-		if (rt.kind != nkind.N_TTAGGED) { return nil; };
-		return rt.list;
+		if (rtyp.kind != nkind.N_TTAGGED) { return nil; };
+		return rtyp.list;
 	};
 	// Plain call: declared return type is the local's type.
-	return rt;
+	return rtyp;
 };
 
 // letslotsize — slot size for a `let` binding. Like slotsize, but
@@ -2339,8 +2339,8 @@ fn matchscrutt(c: *cgen, scrut: *node) *node = {
 				};
 			};
 			if (cnm.len > 0) {
-				let rt: *node = fnretlookupmod(c, cnm, cmod);
-				if (rt != nil) { return resolvetagged(c, rt); };
+				let rtyp: *node = fnretlookupmod(c, cnm, cmod);
+				if (rtyp != nil) { return resolvetagged(c, rtyp); };
 			};
 		};
 		return nil;
@@ -2534,9 +2534,9 @@ export fn exprfloatkind(c: *cgen, n: *node) i32 = {
 			if (n.lhs.kind == nkind.N_IDENT) { nm = n.lhs.str; };
 		};
 		if (nm.len > 0) {
-			let rt: *node = fnretlookup(c, nm);
-			if (isf32type(c, rt)) { return 1; };
-			if (isfloattype(c, rt)) { return 2; };
+			let rtyp: *node = fnretlookup(c, nm);
+			if (isf32type(c, rtyp)) { return 1; };
+			if (isfloattype(c, rtyp)) { return 2; };
 		};
 		return 0;
 	};
@@ -3055,9 +3055,9 @@ fn rhstaggedabicall(c: *cgen, src: *node) bool = {
 				};
 			};
 			if (calleename.len > 0) {
-				let rt: *node = fnretlookupmod(c, calleename, cmod);
-				if (rt != nil) {
-					if (istaggedtype(c, rt)) { return true; };
+				let rtyp: *node = fnretlookupmod(c, calleename, cmod);
+				if (rtyp != nil) {
+					if (istaggedtype(c, rtyp)) { return true; };
 				};
 			};
 		};

selfhost/cmd/wcc/check.ww

@@ -812,15 +812,15 @@ fn astoffset(c: *checker, dot: *node) i64 = {
 	if (dot.kind != nkind.N_DOT) { return -1i64; };
 	let recv: *node = scruttype(c, dot.lhs);
 	if (recv == nil) { return -1i64; };
-	let rt: *node = resolvealias(c, unwrapbang(recv));
-	if (rt == nil) { return -1i64; };
-	if (rt.kind == nkind.N_TPTR) {
-		rt = resolvealias(c, unwrapbang(rt.lhs));
+	let rtyp: *node = resolvealias(c, unwrapbang(recv));
+	if (rtyp == nil) { return -1i64; };
+	if (rtyp.kind == nkind.N_TPTR) {
+		rtyp = resolvealias(c, unwrapbang(rtyp.lhs));
 	};
-	if (rt == nil) { return -1i64; };
-	if (rt.kind != nkind.N_TSTRUCT) { return -1i64; };
+	if (rtyp == nil) { return -1i64; };
+	if (rtyp.kind != nkind.N_TSTRUCT) { return -1i64; };
 	let off: i64 = 0i64;
-	let f: *node = rt.list;
+	let f: *node = rtyp.list;
 	for (f != nil) {
 		if (f.kind == nkind.N_TFIELD) {
 			let fa: i64 = astalign(c, f.lhs);

selfhost/cmd/wcc/mem.ww

@@ -11,6 +11,7 @@
 package wcc;
 
 import os;
+import rt;
 
 def ALIGN: u64       = 16u64;
 def INIT_CHUNK: u64  = 65536u64;
@@ -30,8 +31,8 @@ fn roundup(n: u64, a: u64) u64 = {
 };
 
 export fn newarena() *arena = {
-	let a: *arena = os.alloc(ARENA_SZ): *arena;
-	a.buf = os.alloc(INIT_CHUNK): *u8;
+	let a: *arena = rt.alloc(ARENA_SZ): *arena;
+	a.buf = rt.alloc(INIT_CHUNK): *u8;
 	a.off = 0u64;
 	a.cap = INIT_CHUNK;
 	a.next = nil;
@@ -48,14 +49,14 @@ fn grow(a: *arena, need: u64) bool = {
 	if (want > MAX_CHUNK) { want = MAX_CHUNK; };
 	if (want < need) { return false; };	// single allocation too big
 
-	let old: *arena = os.alloc(ARENA_SZ): *arena;
+	let old: *arena = rt.alloc(ARENA_SZ): *arena;
 	old.buf = a.buf;
 	old.off = a.off;
 	old.cap = a.cap;
 	old.next = a.next;
 	old.total = 0u64;
 
-	a.buf = os.alloc(want): *u8;
+	a.buf = rt.alloc(want): *u8;
 	a.off = 0u64;
 	a.cap = want;
 	a.next = old;