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— the issue is superseded and its behavior is covered by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/backend/llvm/ops.zig

@@ -1299,7 +1299,7 @@ pub const Ops = struct {
             if (st_kind == c.LLVMVectorTypeKind or st_kind == c.LLVMScalableVectorTypeKind) {
                 // Vector lane address: GEP [0, lane] into the in-memory vector,
                 // yielding a pointer to the lane element for a scalar store
-                // (vector lane assignment — issue 0086). Mirrors how the read
+                // (vector lane assignment). Mirrors how the read
                 // path extracts a lane; here we address it for a store.
                 var indices = [_]c.LLVMValueRef{
                     c.LLVMConstInt(self.e.cached_i64, 0, 0),