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— the issue is superseded and its behavior is covered by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

@@ -11,11 +11,11 @@ const sema = @import("sema.zig");
 const types = @import("types.zig");
 const Type = types.Type;
 
-// issue 0089 — the backtick raw escape must hold in BOTH classifiers. A raw
+// the backtick raw escape must hold in BOTH classifiers. A raw
 // reserved-name type reference (`` `s2 ``) resolves to the user-declared type,
 // while a BARE `s2` stays the builtin int. Before the fix sema's
 // `resolveTypeNode` ran `Type.fromName` first and ignored `is_raw`, so the
-// editor index would show the builtin for backtick code (the issue-0083
+// editor index would show the builtin for backtick code (the
 // two-resolver divergence applied to raw types).
 test "sema: backtick raw type reference resolves to the user type; bare stays builtin" {
     var arena = std.heap.ArenaAllocator.init(std.testing.allocator);
@@ -85,7 +85,7 @@ test "sema: a raw struct-field annotation resolves to the user type; bare stays
     try std.testing.expectEqual(@as(u8, 8), b_ty.?.unsigned);
 }
 
-// ── issue 0089: raw provenance through sema's COMPOUND type metadata ────────
+// ── raw provenance through sema's COMPOUND type metadata ────────
 //
 // The direct-case fix (above) only covered a bare `` `s2 `` reference. A
 // COMPOUND raw type (`*`s2`, `?`s2`, `[N]`s2`, …) stores its inner name as a