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— the issue is superseded and its behavior is covered by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/interp.test.zig

@@ -808,7 +808,7 @@ test "comptime: type_eq builtin on type_tag values" {
 // ── Test: reflectTypeId reads an Any's runtime TYPE-TAG, not its payload ──
 // A reflection builtin on an Any must report the type OF a held value (the
 // tag) and only read the payload when the Any holds a Type value (tag ==
-// `.any`). Regression for issue 0090 (attempt 3): a boxed value like
+// `.any`). Regression: a boxed value like
 // `av : Any = 6` (`{ tag = s64, value = 6 }`) must resolve to `s64`, NOT
 // `types[6]` (`u8`).
 test "reflect: reflectTypeId branches on the Any tag" {