wwstage: expand spread variants in match check + size them off flattened members (#209)

The wwstage checker walked a match's raw AST variant list and never expanded a ...inner spread variant, so it rejected fmt's match over field = (...formattable | *mods) ('not a variant of scrutinee'). cstage's resolve_type flattens the spread at type-build. Mirror that in the AST exhaustiveness walk (casevariantin + a recursive checkvariantcovered): when a variant resolves to N_TTAGGED via a spread, recurse into its members. Additive + spread-gated -- typeeqast / casevariantpairmatch (#13) / casecovers untouched, so non-spread matches and 990-997 byte-id are unaffected. Also size a spread N_TTAGGED off each flattened member (mirror cstage check.c), dropping the inner union tag word (field 40B to 32B). Closes the #209 CHECKER reject; full fmt-byte-id still awaits cgen cluster #226 (io.read nominal-remap) + #227 (spread-widen ABI), so fmt tests stay cstage-only with retargeted comments. Adds test 792; regenerates w6c/wwdump combined.ww.

test/wcc/777_fmt_handle_run.c

@@ -16,18 +16,19 @@
  * (the io.stream arm) and reads back via memio.string — the two-arm
  * coverage #5 graduated.
  *
- * Cstage-only per row (no STAGE_WW, no byte_id) — pre-existing wwstage
- * bug #209 (sibling of #190): the wwstage checker bails `case: not a
- * variant of scrutinee (X)` + `match: variant not handled
- * (formattable)` on fmt's match-on-formattable arms whenever a
- * downstream probe `import fmt;`s the package. The bug bites identically
- * — `fmt.errorln("hi")` from a probe trips the same trace — and is
- * pre-existing (reproduces on master). Every fmt test is cstage-only
- * (970 fmttest runs the @test fixture under cstage `ww run` only); 995
- * self-rebuild dodges it because no selfhost main pulls err.ww
- * transitively, so wwstage never compiles fmt in the bootstrap. Fix =
- * one-class checker repair, out of #5 commit-2 scope; byte-id graduates
- * with #209 close.
+ * Cstage-only per row (no STAGE_WW, no byte_id). #209 (the wwstage
+ * checker bail `case: not a variant of scrutinee` + `match: variant not
+ * handled (formattable)` on fmt's spread-union match-arms) is now CLOSED
+ * — wwstage compiles fmt. The residual blocker is a SEPARATE pre-existing
+ * cgen cluster surfaced once fmt actually codegens under wwstage:
+ *   - #226: io.read's error widening (cgwidentagremap) loses the
+ *     io.eof/io.error NAMED tinfo identity under fmt-presence
+ *     (flatvariantidxt → -1 → tag collapses to 0); cs!=ww whole-file, so
+ *     byte-id fails even though io.read is unused here (it's emitted
+ *     regardless). The #10/#218/tinfo-lossy-nominal family.
+ *   - #227: spread-union widen + return-ABI, broken in BOTH stages.
+ * 995 self-rebuild dodges all of this because no selfhost main pulls fmt
+ * transitively. Byte-id graduates when #226 (+#227) land.
  *
  *   row                  | what it pins
  *   ---------------------+--------------------------------------
@@ -293,8 +294,8 @@ run_row(const char *driver, const char *cwd, const struct row *r, int seq)
 /* asm_byte_identical — diff cstage vs wwstage .s. Parallel trees so
  * ww_ww writing intermediates next to the source doesn't clobber the
  * cstage .s (CLAUDE.md rule 14 phase split). Mirror of 776's. Unused
- * while every row is cstage-only (#209), kept for the byte-id
- * graduation once #209 closes. */
+ * while every row is cstage-only (#226/#227; #209 is closed), kept for
+ * the byte-id graduation once those cgen folds land. */
 static int
 asm_byte_identical(const char *cdrv, const char *wdrv, const char *cwd,
     const struct row *r, int seq)
@@ -356,7 +357,7 @@ main(void)
 	int total = 0, fail = 0;
 	int wwpresent = (access(wdrv, X_OK) == 0);
 	int seq = 0;
-	(void)asm_byte_identical;	/* cstage-only until #209 closes */
+	(void)asm_byte_identical;	/* cstage-only until #226/#227 land */
 
 	for (int i = 0; i < n; i++) {
 		if (rows[i].stage_mask & STAGE_CS) {