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— the issue is superseded and its behavior is covered by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

@@ -45,7 +45,7 @@ pub fn constArrayLiteral(self: *Lowering, elements: []const *const Node, array_t
 /// Try to convert a single AST expression into a compile-time ConstantValue.
 /// `expected_ty` is the destination element/field type — it lets aggregate
 /// leaves (struct literals, nested arrays) serialize with the correct shape
-/// rather than collapsing to null (issue 0080). Returns null if the
+/// rather than collapsing to null. Returns null if the
 /// expression is not constant-foldable here.
 pub fn constExprValue(self: *Lowering, expr: *const Node, expected_ty: TypeId) ?inst_mod.ConstantValue {
     return switch (expr.data) {
@@ -66,7 +66,7 @@ pub fn constExprValue(self: *Lowering, expr: *const Node, expected_ty: TypeId) ?
         .undef_literal => .zeroinit,
         // A `null` in a pointer (or optional-pointer) field is a
         // compile-time constant: the zero pointer. Without this arm the
-        // aggregate is wrongly rejected as non-constant (issue 0081).
+        // aggregate is wrongly rejected as non-constant.
         .null_literal => .null_val,
         .unary_op => |uo| switch (uo.op) {
             .negate => switch (uo.operand.data) {
@@ -80,7 +80,7 @@ pub fn constExprValue(self: *Lowering, expr: *const Node, expected_ty: TypeId) ?
         .struct_literal => |sl| self.constStructLiteral(&sl, expected_ty),
         // An enum tag as an aggregate leaf (`[2]Color = .[.green, .blue]`, or
         // an enum field inside a global struct) serializes to its tag int
-        // against the leaf's declared enum type (issue 0082).
+        // against the leaf's declared enum type.
         .enum_literal => |el| self.constEnumLiteral(&el, expected_ty, expr.span),
         else => null,
     };
@@ -92,7 +92,7 @@ pub fn constExprValue(self: *Lowering, expr: *const Node, expected_ty: TypeId) ?
 /// (`enum { a; b :: 5; }`); the enum's backing width is applied by the
 /// const emitters via the destination type's LLVM type. Plain enums only —
 /// a tagged-union or non-enum destination is diagnosed loudly rather than
-/// silently zero-initialized (issue 0082).
+/// silently zero-initialized.
 pub fn constEnumLiteral(self: *Lowering, el: *const ast.EnumLiteral, ty: TypeId, span: ast.Span) ?inst_mod.ConstantValue {
     if (!ty.isBuiltin()) {
         const info = self.module.types.get(ty);
@@ -543,7 +543,7 @@ pub fn lowerComptimeCall(self: *Lowering, fd: *const ast.FnDecl, call_node: *con
     // its return type + anything it `#insert`s, e.g. `build_format` / `out`
     // / `emit` inside `std.print` / `log.*`), so those bare names resolve
     // in the defining module's visibility context rather than the call
-    // site's (issue 0106). The call-site ARGS above are deliberately lowered
+    // site's. The call-site ARGS above are deliberately lowered
     // BEFORE this, 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 sourceless body keeps the
@@ -848,7 +848,7 @@ const ConstAuthor = union(enum) {
 ///
 /// - **own-wins**: the querying module's OWN const author is selected outright.
 /// - else the FLAT-import-reachable const authors: exactly one → it; ≥2 distinct
-///   → `.ambiguous` (issue 0105 / 0760 — never a silent first-/last-wins pick).
+/// → `.ambiguous` (never a silent first-/last-wins pick).
 /// - none visible → `.none` (a namespaced-only const must be qualified `ns.X`;
 ///   a non-const name folds to `.none` too).
 ///