fix(lower): null-FuncId path restores full caller state [0100 F2]

lazyLowerFunction's three exit paths (non-null branch, already-promoted
early return, null-FuncId `ns.fn` qualified-alias branch) each duplicated
the caller-state restore, and the null branch's copy had drifted: it
restored every saved field EXCEPT `block_terminated`. A qualified alias
whose body terminates (e.g. a constant-folded `if true { return ... }`)
leaves `block_terminated = true` after lowerFunction; the null path
returned without resetting it, so the flag leaked into the CALLER's body
lowering and the caller's own trailing statements / `return` were rejected
as dead-after-terminator ("function ... body produces no value").

Fix: collapse the three restores into a single `defer` registered right
after the state is saved, so every exit path restores the identical full
set and the class cannot diverge again. Fields restored on all paths:
current_source_file (F1), scope, func_defer_base, block_terminated (F2),
force_block_value, builder.func/current_block/inst_counter. The
foreign-class / jni-env / pack-mono / inline-return fields already had
their own defers and are unchanged.

Regression: examples/0721-modules-qualified-terminating-callee.sx — a
qualified alias `m.foo` folds `if true { return helper(); }` (helper from
m.sx's own import) and is followed by caller statements + the caller's own
`return 0`. Reports "body produces no value" pre-fix; prints
"terminating-callee: ok" / "after" and exits 0 after. 0719 (collision) and
0720 (F1 own-import visibility) stay green. issues/0100 RESOLVED banner
extended with the F2 follow-up.

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

@@ -69,6 +69,53 @@ Regression: `examples/0720-modules-qualified-own-import.sx` — `calc.compute`
 Reports `'triple' is not visible` on the attempt-1 code; passes after. 0719's
 cross-module dual-`parse` assertion stays green.
 
+## F2 follow-up — null-FuncId path must restore the FULL caller lowering state
+
+The F1 fix patched the **source file** in `lazyLowerFunction`'s null-FuncId
+branch, but that branch still restored only a SUBSET of the caller state the
+non-null branch restores — it omitted `self.block_terminated`. A qualified
+alias whose body terminates (e.g. a constant-folded `if true { return … }`)
+leaves `block_terminated = true` after `lowerFunction`; the null branch then
+returned without resetting it, so the flag leaked into the **caller's** body
+lowering and the caller's own trailing statements / `return` were treated as
+dead-after-terminator:
+
+```
+m :: #import "m.sx";   // m.sx: `#import "helper.sx"; foo :: () -> s64 { if true { return helper(); } return 0; }`
+main :: () -> s32 {
+    x := m.foo();
+    print("after\n");  // dropped
+    return 0;          // → error: body produces no value
+}
+```
+
+**Fix** (`src/ir/lower.zig`): the three exit paths of `lazyLowerFunction` (the
+null-FuncId branch, the already-promoted early return, and the bottom of the
+non-null branch) duplicated the restore, and the null branch's copy drifted.
+They are now collapsed into a **single `defer`** registered right after the
+state is saved, so every exit path restores the identical full set and the
+class can't diverge again. The fields the defer now restores on all paths:
+
+- `current_source_file` (via `setCurrentSourceFile`, which also resyncs
+  `diagnostics.current_source_file`) — F1
+- `scope`
+- `func_defer_base`
+- `block_terminated` — **F2** (was missing on the null path)
+- `force_block_value`
+- `builder.func`
+- `builder.current_block`
+- `builder.inst_counter`
+
+(The `current_foreign_class`, `jni_env_stack_base`, and pack-mono /
+`inline_return_target` fields already had their own `defer`s and apply on all
+paths; they are unchanged.)
+
+Regression: `examples/0721-modules-qualified-terminating-callee.sx` — `m.foo`
+(a qualified alias) folds `if true { return helper(); }` and is followed by
+caller statements + the caller's own `return 0`. Reports `body produces no
+value` on the attempt-2 code; prints `terminating-callee: ok` / `after` and
+exits 0 after. 0719 and 0720 stay green.
+
 ## Symptom
 
 - **Observed:** a program that imports two modules each exporting a
@@ -117,11 +164,15 @@ machinery already existed but was never fed module-qualified entries.
 ## Fix verification
 
 - `zig build` → 0
-- `zig build test` → 0 (incl. LSP corpus sweep, 472 examples)
-- `bash tests/run_examples.sh` → 455 passed, 0 failed
+- `zig build test` → 0 (incl. LSP corpus sweep, 473 examples; 397/397 tests)
+- `bash tests/run_examples.sh` → 456 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.
+- `examples/0721-modules-qualified-terminating-callee.sx`: `body produces no
+  value` on the attempt-2 code, passes after the F2 fix.
 
 Regression tests: `examples/0719-modules-cli-and-json.sx` (collision),
-`examples/0720-modules-qualified-own-import.sx` (F1 own-import visibility).
+`examples/0720-modules-qualified-own-import.sx` (F1 own-import visibility),
+`examples/0721-modules-qualified-terminating-callee.sx` (F2 terminating
+qualified callee — caller state transparency).