lib: α-batch-2 rt.malloc → alloc([], N)! (path/shlex/fmt/ostest)

Phase 0 #8 second α-batch. 7 sites: lib/path/path.ww `join` ×4,
lib/shlex/shlex.ww `dupstr`, lib/fmt/fmt.ww `asprintf` tight-copy,
lib/os/ostest.ww `test_alloc_free_roundtrip`. Same dup-pilot pattern
(4c07ef0, 47918d3): `alloc([], N)!` + `buf.len = N;` +
`return strings.frombytes(buf);`.

Side effects:
- path/shlex/fmt: import switches `rt` → `strings` (callers now
  reference `strings.frombytes`, not `rt.malloc` direct).
- ostest.ww: `import rt;` retained — the alloc builtin lowers to
  `CALL malloc(SB)` which resolves via rt's @symbol("rt_malloc")
  decl. Other files reach rt transitively via `import strings`;
  ostest only imports os, so it needs the explicit rt import.
- shlex stale comment "avoid strings dep" stripped — strings is now
  in scope.

Verified make test 132/132 + 995_self_rebuild byte-identity.
Advances #43.

lib/path/path.ww

@@ -5,7 +5,7 @@
 
 package path;
 
-import rt;
+import strings;
 
 def SEP: u8 = 47u8;	// '/'
 
@@ -95,31 +95,25 @@ export fn extension(p: str) str = {
 // two-arg join (no variadic).
 export fn join(a: str, b: str) str = {
 	if (abs(b)) {
-		let buf: *u8 = rt.malloc(b.len: u64): *u8;
+		let buf: []u8 = alloc([], b.len: u64)!;
 		let i: i32 = 0;
 		for (i < b.len) { buf[i] = b[i]; i += 1; };
-		let r: str;
-		r.ptr = buf;
-		r.len = b.len;
-		return r;
+		buf.len = b.len;
+		return strings.frombytes(buf);
 	};
 	if (a.len == 0) {
-		let buf: *u8 = rt.malloc(b.len: u64): *u8;
+		let buf: []u8 = alloc([], b.len: u64)!;
 		let i: i32 = 0;
 		for (i < b.len) { buf[i] = b[i]; i += 1; };
-		let r: str;
-		r.ptr = buf;
-		r.len = b.len;
-		return r;
+		buf.len = b.len;
+		return strings.frombytes(buf);
 	};
 	if (b.len == 0) {
-		let buf: *u8 = rt.malloc(a.len: u64): *u8;
+		let buf: []u8 = alloc([], a.len: u64)!;
 		let i: i32 = 0;
 		for (i < a.len) { buf[i] = a[i]; i += 1; };
-		let r: str;
-		r.ptr = buf;
-		r.len = a.len;
-		return r;
+		buf.len = a.len;
+		return strings.frombytes(buf);
 	};
 	// Trim trailing '/' from a; b never starts with '/' here (checked
 	// above via abs(b)).
@@ -129,14 +123,12 @@ export fn join(a: str, b: str) str = {
 		an -= 1;
 	};
 	let total: i32 = an + 1 + b.len;
-	let buf: *u8 = rt.malloc(total: u64): *u8;
+	let buf: []u8 = alloc([], total: u64)!;
 	let i: i32 = 0;
 	for (i < an) { buf[i] = a[i]; i += 1; };
 	buf[an] = SEP;
 	let j: i32 = 0;
 	for (j < b.len) { buf[an + 1 + j] = b[j]; j += 1; };
-	let r: str;
-	r.ptr = buf;
-	r.len = total;
-	return r;
+	buf.len = total;
+	return strings.frombytes(buf);
 };