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/cgen.ww

@@ -165,6 +165,8 @@ type cgen = struct {
 	looptop: i32,
 	loopendbuf:  *str,	// stack of end labels for break
 	loopcontbuf: *str,	// stack of cont labels for continue
+	yieldtop: i32,
+	yieldbuf:    *str,	// stack of match end labels for yield
 };
 
 fn cgeninit(c: *cgen, a: *arena) void = {
@@ -179,6 +181,8 @@ fn cgeninit(c: *cgen, a: *arena) void = {
 	c.looptop = 0;
 	c.loopendbuf  = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
 	c.loopcontbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
+	c.yieldtop = 0;
+	c.yieldbuf = amalloc(a, (LOOP_MAX: u64) * 16u64): *str;
 };
 
 // localalloc — append a slot for `name` without dedup. Used for