refactor(ffi-linkage): Phase 9.3-src — purge 'foreign' from src/ comments + a user-facing diagnostic

Reword every 'foreign' comment to the extern/runtime-class vocabulary matching the
renamed identifiers (foreign call→extern call, foreign class→runtime class, foreign
path→runtime path, the #foreign-literal comment mentions → extern, etc.). Also fixes
two USER-FACING issues: the 'expected … #foreign … after type annotation' parse error
no longer advertises the removed keyword, and the Android 'no #jni_main' help
diagnostic now shows '#jni_class(…) extern' instead of the rejected '#foreign
#jni_class'. Removed the now-dead prefix-#foreign-vs-postfix conflict branch in
parseRuntimeClassDecl (the caller rejects #foreign before it runs).

src/ now contains 'foreign' ONLY in the hash_foreign token machinery + its 4
rejection messages — the deprecation mechanism (kept per the 9.0 recommendation; the
message MUST name #foreign to guide migration). Snapshot-neutral; suite green
(646 corpus / 444 unit, 0 failed).

src/ir/lower/nominal.zig

@@ -257,7 +257,7 @@ pub fn shadowNominalId(self: *Lowering, name_id: types.StringId) u32 {
 }
 
 /// TRUE iff `name` is authored AS A NAMED TYPE (struct / enum / union /
-/// error-set / protocol / foreign class) by ≥2 DISTINCT modules in the import
+/// error-set / protocol / runtime class) by ≥2 DISTINCT modules in the import
 /// raw facts — the authoritative same-name-shadow signal (the only case where
 /// distinct `nominal_id`s are needed). Module distinctness is by LEXICALLY
 /// NORMALIZED path: one logical file reached through several spellings