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/778_bufio_vstream_run.c

@@ -15,9 +15,12 @@
  *     scanline / finish.
  *
  * Each row imports os + bufio + memio + io (NOT fmt — bufio doesn't
- * transitively pull fmt, so #209's formattable-match bail doesn't
- * propagate; both stages run on every row, cs.s==ww.s — bufio is NOT
- * compiler-embedded, so these byte-id rows are its ONLY byte-id cover).
+ * transitively pull fmt, so it dodges the fmt-presence cgen cluster
+ * #226 (io.read error-remap nominal-identity) + #227 (spread-union
+ * widen/return-ABI) that blocks the fmt-importing tests' byte-id; both
+ * stages run on every row, cs.s==ww.s — bufio is NOT compiler-embedded,
+ * so these byte-id rows are its ONLY byte-id cover). (#209, the wwstage
+ * formattable match-arm CHECKER bail, is now closed.)
  *
  *   row                       | what it pins
  *   --------------------------+--------------------------------------