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— the issue is superseded and its behavior is covered by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

@@ -627,7 +627,7 @@ pub fn lowerPackFnCall(self: *Lowering, fd: *const ast.FnDecl, call_node: *const
     // `target_type`) would cast to the stale target — e.g. `format("…", xx i)`
     // inside a `-> string` fn mis-typed the arg as `string`, monomorphizing
     // `__pack_string` and ABI-coercing the 4-byte int as a 16-byte fat
-    // pointer → memory corruption (issue 0057).
+    // pointer → memory corruption.
     const saved_pack_tt = self.target_type;
     self.target_type = null;
     var pack_refs = std.ArrayList(Ref).empty;
@@ -895,7 +895,7 @@ pub fn monomorphizePackFn(
     // OWN module (E4), so a 2-flat-hop library type named in the signature is
     // bare-visible — mirrors the body pin further down and the
     // `monomorphizeFunction` pin. The comptime call-site args below are
-    // lowered AFTER this restore, in the caller's context (issue 0106).
+    // lowered AFTER this restore, in the caller's context.
     const saved_sig_src = self.current_source_file;
     if (fd.body.source_file) |src| self.setCurrentSourceFile(src);
 
@@ -1023,7 +1023,7 @@ pub fn monomorphizePackFn(
     // Pin to the metaprogram's OWN module for the BODY lowering only, so its
     // bare names (and anything it `#insert`s — e.g. `build_format` / `out` /
     // `emit` inside `std.print`) resolve in the defining module's visibility
-    // context, not the call site's (issue 0106). The comptime-param call-site
+    // context, not the call site's. The comptime-param call-site
     // args above were deliberately lowered FIRST, in the caller's context.
     // Mirrors `lowerFunctionBodyInto`, which switches to `func.source_file`;
     // the defining path is stamped on the body node by `resolveImports`. A