cleanup: drop resolved-issue citations from src comments

Sweep all src/**.zig comments that cite resolved issues (issue NNNN /
fix-NNNN / KB-N): the invariant or mechanism each comment states is
kept; the historical citation is dropped, per the no-conclusion-comments
rule. Pure-history parentheticals are removed outright. References to
the 16 still-open issues (0030, 0041-0056) are untouched, as are test
NAMES carrying regression provenance (matching the sanctioned
"Regression (issue NNNN)" example-header convention).

Also removes the issues/0019-import-non-transitive-c-scope/ fixture dir
— the issue is superseded and its behavior is covered by
examples/0706-modules-import-non-transitive.sx (the .md writeup stays).
issues/0030's repro .sx stays: that issue is an open feature request.

Gate: zig build OK; zig build test 426/426; run_examples 541/0; zero
expected/ snapshot churn.

src/ir/lower/coerce.zig

@@ -543,8 +543,8 @@ pub fn zeroValue(self: *Lowering, ty: TypeId) Ref {
     };
 }
 
-/// Emit the unified non-integral float→int narrowing diagnostic (F0.11 /
-/// issue 0095). ONE wording, ONE place: every site that rejects an implicit
+/// Emit the unified non-integral float→int narrowing diagnostic (F0.11).
+/// ONE wording, ONE place: every site that rejects an implicit
 /// narrowing of a non-integral compile-time float to an integer type calls
 /// this, so the message + fix-it stay identical across the typed-binding
 /// coerce arm, the field/param-default sites, the typed-const path, and the