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— the issue is superseded and its behavior is covered by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

@@ -1,4 +1,4 @@
-// Tests for imports.zig — flat-import name-resolution data retention (fix-0102a).
+// Tests for imports.zig — flat-import name-resolution data retention.
 
 const std = @import("std");
 const ast = @import("ast.zig");
@@ -179,7 +179,7 @@ test "imports: module_decls retains same-name cross-module fns; flat_import_grap
 }
 
 // Mixed collision: a.sx authors `Widget` as a STRUCT (non-fn), b.sx authors it
-// as a FUNCTION. fix-0102a must NOT let the function-author retention shift the
+// as a FUNCTION. The function-author retention must NOT shift the
 // merged scope — first-wins keeps a.sx's struct and drops b.sx's function.
 // (The fn author may still be indexed in `module_decls`; resolution is what
 // must be untouched.)