fix(lower): qualified alias lowers in its own source context [0100 F1]

The 0100 identity fix registers a namespaced import's own functions under a
module-qualified name (ns.fn) in fn_ast_map WITHOUT an eager declareFunction,
so the alias is lowered through lazyLowerFunction's null-FuncId lowerFunction
path. That path had no Function.source_file to restore (the non-null path does
setCurrentSourceFile(func.source_file)), so the alias lowered in the CALLER's
visibility context. A qualified function that called a helper from its OWN
module's flat import was then rejected "not visible".

Fix:
- ProgramIndex.qualified_fn_source maps each ns.fn alias to its declaring
  source file, populated in registerQualifiedFn (current_source_file is
  pinned to the decl's source by registerNamespaceQualifiedFns).
- lazyLowerFunction's null-FuncId branch restores that source before
  lowerFunction, so ns.fn's body lowers in its own module's context and its
  intra-module / own-import callees resolve.
- lowerFunction records Function.source_file = current_source_file on the
  freshly-begun function (matching declareFunction), so the lowered alias
  carries its own module for diagnostics/emit.

Regression: examples/0720-modules-qualified-own-import.sx — calc.compute (a
qualified alias) calls triple/base from calc.sx's own flat import; reports
"'triple' is not visible" on the attempt-1 code, passes after. 0719's
cross-module dual-parse assertion stays green. issues/0100 RESOLVED banner
extended with the F1 follow-up.

issues/0100-cross-module-same-name-fn-lowering-collision.md

@@ -38,6 +38,37 @@ and `std.json` under distinct namespaces and calls both `cli.parse`
 (dispatch) and `json.parse` (document read), asserting correct results.
 Panics on pre-fix code; passes after.
 
+## F1 follow-up — qualified alias must lower in its own source context
+
+The identity fix above registered `ns.fn` in `fn_ast_map` WITHOUT an eager
+`declareFunction`, so the qualified alias is lowered through
+`lazyLowerFunction`'s **null-FuncId** `lowerFunction` path — which had no
+`Function.source_file` to restore (the non-null path does
+`setCurrentSourceFile(func.source_file)`). The alias therefore lowered in the
+**caller's** visibility context, and a qualified function calling a helper
+from **its own module's flat import** was rejected:
+
+```
+m :: #import "m.sx";            // m.sx: `#import "helper.sx"; foo :: () { helper() }`
+main :: () -> s32 { print("{}\n", m.foo()); 0 }   // → 'helper' is not visible
+```
+
+**Fix** (`src/ir/program_index.zig`, `src/ir/lower.zig`):
+
+- New `ProgramIndex.qualified_fn_source` (qualified name → declaring source
+  file), populated in `registerQualifiedFn` from the decl's own source.
+- `lazyLowerFunction`'s null-FuncId branch restores that source via
+  `setCurrentSourceFile` before calling `lowerFunction`, so `ns.fn`'s body
+  lowers in its own module's context and its own-import callees resolve.
+- `lowerFunction` now records `Function.source_file = current_source_file`
+  on the freshly-begun function (matching `declareFunction`), so the lowered
+  alias carries its own module for diagnostics/emit.
+
+Regression: `examples/0720-modules-qualified-own-import.sx` — `calc.compute`
+(a qualified alias) calls `triple` / `base` from calc.sx's own flat import.
+Reports `'triple' is not visible` on the attempt-1 code; passes after. 0719's
+cross-module dual-`parse` assertion stays green.
+
 ## Symptom
 
 - **Observed:** a program that imports two modules each exporting a
@@ -86,8 +117,11 @@ machinery already existed but was never fed module-qualified entries.
 ## Fix verification
 
 - `zig build` → 0
-- `zig build test` → 0 (incl. LSP corpus sweep, 471 examples)
-- `bash tests/run_examples.sh` → 454 passed, 0 failed
+- `zig build test` → 0 (incl. LSP corpus sweep, 472 examples)
+- `bash tests/run_examples.sh` → 455 passed, 0 failed
 - `examples/0719-modules-cli-and-json.sx`: panics pre-fix, passes post-fix.
+- `examples/0720-modules-qualified-own-import.sx`: `'… is not visible'` on
+  the attempt-1 code, passes after the F1 fix.
 
-Regression test: `examples/0719-modules-cli-and-json.sx`.
+Regression tests: `examples/0719-modules-cli-and-json.sx` (collision),
+`examples/0720-modules-qualified-own-import.sx` (F1 own-import visibility).