wcc/ww: .wwi export-data producer + check_exported_type (#22 M2)

New `w6c -I <out.wwi>` flag (both stages) writes a re-parseable
ww-prototype rendering of a package's EXPORTED surface. M2 dead-code:
nothing consumes .wwi yet (combined.ww stays the live path); the flag is
off on every existing invocation, so the 990-997 byte-id gates and all
prior tests are unperturbed.

The unparse walks the AST type-expr subtree (N_T* nodes), not the
tinfo Type* (which collapses nominal pkg.Name identity). Deterministic
output: package line, byte-sorted imports, byte-sorted decls — a pure
function of the exported API. cmd/w6c/wwi.c + selfhost/cmd/wcc/wwi.ww
emit byte-identical .wwi (new cross-stage byte-id substrate, rule 10).

check_exported_type (drew) rides the producer entry, flag-gated: an
exported signature naming a non-exported nominal is loud-rejected before
any byte is written, identically on both stages. Ports harec
check.c:4092-4168, recursing the type-AST and gating on the resolved
SK_TYPE sym's decl export flag (Sym.exported is vestigial in both
stages; the predeclared synthetic `nomem` decl carries no source
position and is treated as a builtin leaf — cstage parity).

Two wwstage checker AST-mutations are normalized to cstage's pristine
view for byte-id: the N_TPARAM tuple-element wrapper (unwrapped) and the
variadic `T...`→`[]T` param desugar (peeled).

Gate 989_m2wwi_run: ascii/strings/getopt each produce a .wwi that
re-parses (wwdump -a) and is cs==ww byte-identical; a private-type-leak
fixture is rejected identically by both stages (non-vacuous check).

selfhost/cmd/w6c/main.ww

@@ -23,6 +23,7 @@ import typ;
 import sym;
 import check;
 import cgen;
+import wwi;
 
 fn cstreq(a: *u8, lit: str) bool = {
 	let n: u64 = lit.len: u64;
@@ -79,6 +80,7 @@ fn slurp(path: *u8) (*u8, u64) = {
 export fn main(argc: i32, argv: **u8) i32 = {
 	let src: *u8 = nil;
 	let out: *u8 = nil;
+	let wwiout: *u8 = nil;		// -I <out.wwi>: M2 export-data producer
 	let testmode: i32 = 0i32;	// #15: `-T` test-mode
 
 	let i: i32 = 1;
@@ -92,6 +94,14 @@ export fn main(argc: i32, argv: **u8) i32 = {
 				return 2;
 			};
 			out = argv[i];
+		} else { if (cstreq(a, "-I")) {
+			i += 1;
+			if (i >= argc) {
+				let m: str = "w6c: -I requires arg\n";
+				os.write(2, m.ptr, m.len: u64);
+				return 2;
+			};
+			wwiout = argv[i];
 		} else { if (cstreq(a, "-T")) {
 			testmode = 1i32;
 		} else { if (a[0u64] == 45u8) {
@@ -105,7 +115,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 				return 2;
 			};
 			src = a;
-		}; }; };
+		}; }; }; };
 		i += 1;
 	};
 
@@ -175,6 +185,13 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	checkfile(&ck, f);
 	if (ck.errs > 0) { return 1; };
 
+	// M2 export-data: write the `.wwi` after a clean check, before cgen.
+	// Dead on the live path (no existing invocation passes -I); the
+	// producer's check_exported_type may reject a dangling export.
+	if (wwiout != nil) {
+		if (wwiemit(&ck, f, pathstr(wwiout)) != 0) { return 1; };
+	};
+
 	let cg: cgen;
 	cgeninit(&cg);
 	cgfile(&cg, f);