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— the issue is superseded and its behavior is covered by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

@@ -930,7 +930,7 @@ pub const LLVMEmitter = struct {
                     .vtable => c.LLVMConstNull(llvm_ty), // placeholder — initialized in initVtableGlobals after function declarations
                     // A top-level null-pointer global (`p : *s64 = null;`) and a
                     // zero-initialized global both emit as the all-zero constant
-                    // of the global's type (issue 0081).
+                    // of the global's type.
                     .null_val, .zeroinit => c.LLVMConstNull(llvm_ty),
                     .undef => c.LLVMGetUndef(llvm_ty),
                     // func_map is empty in Pass 0 (functions are declared in
@@ -2527,7 +2527,7 @@ pub const LLVMEmitter = struct {
                     break :blk c.LLVMConstNull(elem_ty);
                 },
                 // A null pointer field and a zero-initialized field both emit as
-                // the all-zero constant of the leaf type (issue 0081).
+                // the all-zero constant of the leaf type.
                 .null_val, .zeroinit => c.LLVMConstNull(elem_ty),
                 .undef => c.LLVMGetUndef(elem_ty),
                 // Vtable constants are only ever produced for top-level protocol