wcc: match-as-expression with yield

`match (e) { ... }` can now sit in expression position, with each
arm using `yield expr;` to produce the match's value:

    let v = match (r) {
    case let n: i32 => yield n + 1;
    case let s: str => yield s.len: i32 + 100;
    };

TK_YIELD keyword + N_YIELD AST node, both appended at the tail of
their enums to keep prior numeric values byte-stable for the
wwdump-diff gates.

Checker: cexpr for N_MATCH walks each arm's body looking for the
first N_YIELD; the match's type is the unified yield type (or
ty_void if no yield, preserving the statement-form semantics).
Mismatched arm yields are flagged.

Cgen: a yield-target stack (separate from the loop break stack)
holds each enclosing match's end label. N_YIELD evaluates its
expression into AX (and BX for str) and JMPs to the topmost entry.
cgmatch pushes its end label on entry and pops on exit.

Selfhost mirror: lib/ww/lex/tok.ww kwtab+name, lib/ww/ast.ww
N_YIELD def+print, lib/ww/parse/stmt.ww yield-stmt; selfhost cgen
adds a yieldbuf to the cgen struct and a cgyield helper. Verified
end-to-end: a yield-using program compiled via the wwstage cgen
matches the C-cgen build's exit code.

cmd/wcc/ww.h

@@ -177,6 +177,7 @@ typedef enum {
 	 * the selfhost wwdump-diff test (990) is byte-sensitive. */
 	TK_IS,		/* Hare-style type test: e is T */
 	TK_VOID,	/* `void` — both a type name and a zero-size value */
+	TK_YIELD,	/* `yield expr;` — value-return from a match arm */
 
 	TK_LAST		/* sentinel for tables */
 } Tkind;
@@ -297,6 +298,8 @@ typedef enum {
 	N_TYPEASSERT,	/* lhs as T  →  T (abort if tag mismatch) */
 	N_VOIDLIT,	/* `void` as expression — zero-size void value */
 	N_TBANG,	/* `!T` — error-flagged type. lhs = inner type. */
+	N_YIELD,	/* `yield expr;` — set the enclosing match's value-
+			 * return and jump to its end label. lhs = value. */
 
 	N_LAST
 } Nkind;