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— the issue is superseded and its behavior is covered by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

@@ -189,7 +189,7 @@ pub fn lowerLambda(self: *Lowering, lam: *const ast.Lambda) Ref {
     // `defer`s, not the enclosing function's. Open a fresh defer window
     // (like `lowerFunction`/`monomorphizeFunction`) and restore on exit —
     // otherwise lowering a closure literal inside a `defer` body re-enters
-    // the enclosing function's defer drain (infinite recursion — issue 0073).
+    // the enclosing function's defer drain (infinite recursion).
     const saved_func_defer_base = self.func_defer_base;
     const saved_defer_len = self.defer_stack.items.len;
     defer {