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— the issue is superseded and its behavior is covered by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

@@ -779,7 +779,7 @@ pub fn lowerMatch(self: *Lowering, me: *const ast.MatchExpr) Ref {
         if (has_value_merge) {
             // Lower the arm body against the merge's result type so literals
             // (and negated literals) in the arm pick the right width — the
-            // phi operands must all match `result_type` (issue 0066).
+            // phi operands must all match `result_type`.
             const saved_arm_target = self.target_type;
             self.target_type = result_type;
             const maybe_v = self.lowerBlockValue(arm.body);
@@ -790,7 +790,7 @@ pub fn lowerMatch(self: *Lowering, me: *const ast.MatchExpr) Ref {
             // Only materialize a value + branch to the merge when the arm
             // body did NOT diverge. A diverging arm (e.g. `return x`) has
             // already terminated its block; emitting the fallback const
-            // here would land AFTER the terminator (the issue-0057 bug).
+            // here would land AFTER the terminator .
             if (!self.currentBlockHasTerminator()) {
                 var v = maybe_v orelse if (result_type == .string or !result_type.isBuiltin())
                     self.builder.constUndef(result_type)