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.

lib/ww/lex/tok.ww

@@ -108,10 +108,11 @@ def TK_FATARROW: i32 = 81;
 // Appended at the tail (not grouped with the keyword block) so every
 // pre-existing TK_* value stays unchanged — the 990_selfhost test
 // diffs wwdump output against the C side, byte for byte.
-def TK_IS:   i32 = 82;
-def TK_VOID: i32 = 83;
+def TK_IS:    i32 = 82;
+def TK_VOID:  i32 = 83;
+def TK_YIELD: i32 = 84;
 
-def TK_LAST: i32 = 84;
+def TK_LAST:  i32 = 85;
 
 // ---- Pos / Tok --------------------------------------------------------
 //
@@ -183,6 +184,7 @@ export fn kwlookup(p: *u8, n: i32) i32 = {
 	if (streqn(p, "type", n))     { return TK_TYPE; };
 	if (streqn(p, "use", n))      { return TK_USE; };
 	if (streqn(p, "void", n))     { return TK_VOID; };
+	if (streqn(p, "yield", n))    { return TK_YIELD; };
 	return TK_NONE;
 };
 
@@ -225,6 +227,7 @@ export fn tokname(k: i32) str = {
 	if (k == TK_AS)       { return "as"; };
 	if (k == TK_IS)       { return "is"; };
 	if (k == TK_VOID)     { return "void"; };
+	if (k == TK_YIELD)    { return "yield"; };
 	if (k == TK_STATIC)   { return "static"; };
 	if (k == TK_MATCH)    { return "match"; };
 	if (k == TK_CONST)    { return "const"; };