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— the issue is superseded and its behavior is covered by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

@@ -52,7 +52,7 @@ pub const GenericResolver = struct {
         const info = self.l.module.types.get(ty);
         return switch (info) {
             // A nominal type's mangle includes its `nominal_id` when nonzero so two
-            // same-DISPLAY-name authors in different sources (issue 0105) produce
+            // same-DISPLAY-name authors in different sources produce
             // DISTINCT monomorph symbols (`struct_to_string__Box` vs
             // `struct_to_string__Box__n1`) instead of one symbol with conflicting
             // signatures. `nominal_id == 0` (the single-author / structural case)
@@ -103,7 +103,7 @@ pub const GenericResolver = struct {
     }
 
     /// Append a `__n<id>` disambiguator to a nominal type's display name when its
-    /// `nominal_id` is nonzero (a same-name shadow, issue 0105); id 0 returns the
+    /// `nominal_id` is nonzero (a same-name shadow); id 0 returns the
     /// name unchanged so single-author mangling is byte-identical.
     fn mangleNominalName(self: GenericResolver, name: []const u8, nominal_id: u32) []const u8 {
         if (nominal_id == 0) return name;