test: 763 header — cite project #179 by number for the NOT-GATED gap

Round-1 reviewer follow-up to 7031e0d. The brief was explicit
that the test header had to NAME the sibling cgen bug (project
#179, cgmatch flatvariantidx N_TNAME-only gate) — not just
describe its shape. Pre-fix the gap paragraph mentioned the
gate site (selfhost/cmd/wcc/cgenexpr.ww:1512) and the runtime-
masking mechanism, but lacked the literal "#179" handle a
future bisect would grep for. Also folds in the explicit
zero-init-tag-0 masking note (the brief asked for it).

Comment-only — no Makefile, no harness, no row delta.

test/wcc/763_typeeq_fn_ast.c

@@ -41,9 +41,12 @@
  * tag CMPQ because wwstage's cgmatch gates the case-pattern flatvariant
  * lookup on `pat.kind == nkind.N_TNAME` (selfhost/cmd/wcc/cgenexpr.ww:
  * 1512) — N_TPTR(N_TFN) case patterns short-circuit to tag 0. That is
- * a SIBLING cgen-side bug (filed alongside this fold per the brief's
- * "do not sweep" instruction) and orthogonal to the AST-layer
- * typeeqast equality this fold ratifies. The pre/post-fix asymmetry
+ * project #179 (cgmatch flatvariantidx N_TNAME-only gate), a SIBLING
+ * cgen-side bug filed alongside this fold per the brief's "do not
+ * sweep" instruction, and orthogonal to the AST-layer typeeqast
+ * equality this fold ratifies. Runtime is fortuitously OK because
+ * the bare `let v: t;` zero-inits tag 0, masking the wrong-tag-load
+ * the .s divergence would otherwise produce. The pre/post-fix asymmetry
  * THIS PROBE locks is the wwstage CHECKER outcome: pre-fix master
  * red-errors the program (4× errbadcase + match-not-handled); post-
  * fix it compiles to a runnable binary. The runtime exit-code arm