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.

selfhost/cmd/wcc/cgenexpr.ww

@@ -438,6 +438,11 @@ fn cgmatch(c: *cgen, n: *node) void = {
 		};
 	};
 	let endl: str = mklabel(c, "match_end");
+	// Push end label as the yield target for this match's arm bodies.
+	if (c.yieldtop < LOOP_MAX) {
+		c.yieldbuf[c.yieldtop] = endl;
+		c.yieldtop += 1;
+	};
 	let cs: *node = n.list;
 	for (cs != nil) {
 		let nxt: str = mklabel(c, "match_next");
@@ -553,6 +558,7 @@ fn cgmatch(c: *cgen, n: *node) void = {
 		cs = cs.next;
 	};
 	emitlabel(endl);
+	if (c.yieldtop > 0) { c.yieldtop -= 1; };
 	return;
 };