fix(lower): pin defining-module context for pack/comptime metaprograms; drop #insert exemption [stdlib B attempt-3]

ROOT FIX for issue 0106's library-metaprogram half — no exemption.

attempt-2 masked the 0106 fallout with an `in_insert_expansion` flag that
made the visibility adapters fall open during ANY `#insert` expansion,
including a USER's `#insert <expr>` — so a bare reach into a namespaced-only
import from user `#insert` code wrongly compiled (Adi's blocker). The flag
was the wrong shape. This removes it and fixes the real cause.

Root cause: a metaprogram's body (`std.print` / `std.format` / `log.*`,
whose `#insert build_format(fmt)` + `#insert "out(result);"` reference
std-internal bare names) was lowered under the CALL SITE's
`current_source_file`, so those names were policed against the consumer's
imports. Normal functions get this right via `lowerFunctionBodyInto`, which
pins `func.source_file`; the two monomorphizers don't:
  - `monomorphizePackFn`   — bare `print(...)` / `format(...)` (pack path).
  - `lowerComptimeCall`    — namespaced `std.print` / `log.warn` (reached via
                             the field-access `hasComptimeParams` branch).

Fix: both paths now save/set/restore `current_source_file` to the body's
DEFINING module around the BODY lowering only (call-site args stay in the
caller's context). The defining path is stamped onto each function body node
by `resolveImports` (`stampFnBodySource`), mirroring `Function.source_file`.
So library internals resolve in std.sx/log.sx naturally, while a USER's
`#insert <expr>` is still checked in the user's context.

- Exemption GONE: `in_insert_expansion` flag + both adapter fall-open checks
  deleted; `isNameVisible`/`isCImportVisible` are byte-identical adapters.
- New pinned regression: examples/0737-modules-insert-bare-not-visible.sx
  (+ a.sx) — a USER `#insert secret()` into a namespaced-only import errors
  ('secret' is not visible). fail-before exit 0 on the attempt-2 binary /
  pass-after exit 1.
- face #1 (0736) still errors; face #2 (0015/0700/0718/1030) pass again WITH
  NO exemption — the metaprogram body resolves in its own module.
- run_examples 472 -> 473; zig build test 412/412; m3te ios-sim build exit 0.
- issues/0106 RESOLVED banner updated (root cause + no-exemption fix).

examples/0737-modules-insert-bare-not-visible.sx

@@ -0,0 +1,15 @@
+// A bare name inside a USER `#insert <expr>` is visibility-checked in the
+// USER's module, not skipped (regression, issue 0106). `a.sx` is imported only
+// as `m :: #import` — its top-level `secret` is reachable ONLY as `m.secret`.
+// A BARE `secret()` driving a `#insert` must error just like any other bare
+// reference into a namespaced-only import: `#insert` expansion does NOT exempt
+// user-typed code from visibility. (Library metaprograms like `std.print` keep
+// working because their bodies lower in their OWN module's context — see
+// `monomorphizePackFn` / `lowerComptimeCall` pinning `current_source_file` to
+// the body's defining module — not because `#insert` is exempt.)
+m :: #import "0737-modules-insert-bare-not-visible/a.sx";
+
+main :: () -> s32 {
+    #insert secret();
+    0
+}

examples/0737-modules-insert-bare-not-visible/a.sx

@@ -0,0 +1 @@
+secret :: () -> string { "leaked := 1;" }

examples/expected/0737-modules-insert-bare-not-visible.exit

@@ -0,0 +1 @@
+1

examples/expected/0737-modules-insert-bare-not-visible.stderr

@@ -0,0 +1,5 @@
+error: 'secret' is not visible; #import the module that declares it
+  --> examples/0737-modules-insert-bare-not-visible.sx:13:13
+   |
+13 |     #insert secret();
+   |             ^^^^^^

examples/expected/0737-modules-insert-bare-not-visible.stdout

@@ -0,0 +1 @@
+