feat(ffi-linkage): consume export "csym" rename (Phase 2.2) — 1227 green

The define path now honors the optional `export … "csym"` symbol-name
override (gap iii). declareFunction's rename branch fires for `export` too:
the extern stub is declared under the C name and the sx→C mapping recorded
in foreign_name_map. lazyLowerFunction then resolves the stub by that C
name (via foreign_name_map) so the body promotes into the C-named function
— emitting `define @triple_c` instead of `@sx_triple`. sx-side call sites
to the sx name resolve through the same map (verified: 5*5 prints 25).

example/1227 greens: the companion C calls `triple_c` and prints
call_triple(7) = 22. Bare export (1226) is unaffected (no rename → sx
name). Suite green (638 corpus / 443 unit). Phase 2 (`export`) complete.

src/ir/lower/decl.zig

@@ -2124,11 +2124,14 @@ pub fn declareFunction(self: *Lowering, fd: *const ast.FnDecl, name: []const u8)
     const cc: Function.CallingConvention = if (fd.call_conv == .c or is_foreign or is_extern_decl or fd.extern_export == .export_) .c else .default;
 
     // Symbol-name override: `#foreign … "csym"` (foreign_expr.c_name) or the new
-    // `extern … "csym"` (fd.extern_name). Declare under the C name and map the sx
-    // name → C name so call sites resolve to the real symbol.
+    // `extern … "csym"` / `export … "csym"` (fd.extern_name). Declare under the C
+    // name and map the sx name → C name so call sites resolve to the real symbol.
+    // For `export` the stub is later promoted to a real definition (the body
+    // lowers into this C-named function via lazyLowerFunction — Phase 2.2).
+    const is_export_decl = fd.extern_export == .export_;
     const rename_c_name: ?[]const u8 = if (is_foreign)
         fd.body.data.foreign_expr.c_name
-    else if (is_extern_decl)
+    else if (is_extern_decl or is_export_decl)
         fd.extern_name
     else
         null;
@@ -2330,7 +2333,13 @@ pub fn lazyLowerFunction(self: *Lowering, name: []const u8) void {
     // FIRST author of a name owns this slot. A shadowed same-name author is
     // not here (it has no name-keyed slot); it is lowered out-of-line into
     // its OWN FuncId by `lowerRetainedSameNameAuthors`.
-    const name_id = self.module.types.internString(name);
+    // A renamed `export … "csym"` fn was declared under its C symbol name
+    // (declareFunction's rename path), so search for the stub under that name
+    // and promote the body into it. `foreign_name_map` only carries an entry
+    // when a rename was registered; a bare export / normal define keeps its sx
+    // name (Phase 2.2).
+    const search_name = self.foreign_name_map.get(name) orelse name;
+    const name_id = self.module.types.internString(search_name);
     var func_id: ?FuncId = null;
     for (self.module.functions.items, 0..) |func, i| {
         if (func.name == name_id) {