cmd/w6c+selfhost/wcc+lib: route sizeof(str)/sizeof(slice) through SSoT

Audit §1.1/§1.2 cataloged 17 wwstage sites hardcoding 16 for sizeof(str)
and ~10 hardcoding 24 for sizeof(slice), plus 4 cstage str-size sites
and the cstage let_emit_size str/slice arms.  Each new size constant
required ~30 edits in both stages to bump cleanly — task #1 (str → 24B
{ptr,len,cap}) can't land until the literal sweep is done.

Track A — wwstage codegen (selfhost/cmd/wcc/*):

  - check.ww introduces two stateless helpers next to astsize:
    primtypesize(nm)  — primitive-name → byte size (i64; -1 unknown)
    tyslicesize()     — slice-header bytes (i64; 24 today)
    astsize now reads both for its N_TNAME-primitive and N_TSLICE arms,
    so the size(T) fold gets the SSoT for free.
  - cgen.ww, cgenutil.ww, cgenstmt.ww, cgendecl.ww: every `return 16`
    / `esz = 16` / `sz0 = 16` for str, every `return 24` /
    `localadd(c, _, 24, _)` for slice, plus the matching `sz == 16` /
    `sz == 24` / `for (i < 16/24)` gates in the global-let DATAW emit,
    route through primtypesize / tyslicesize.
  - Direct delegation slotsize→astsize would require restructuring
    astsize to drop its *checker dep (resolvealias) — the leaf
    primitive/slice cases factor out cleanly, the alias-chain leaves
    diverge because cgen's aliaslookup/structlookup tables and check's
    scope chain aren't unified yet (§1.8, task #50 follow-up).  Sharing
    the leaf table satisfies the SSoT promise without that refactor.

Track B — cstage (cmd/w6c/cgen.c):

  - let_emit_size's TY_STR/TY_SLICE arms drop the hardcoded 16/24 and
    fall to `(int)u->size` like the existing TY_STRUCT/TUPLE/TAGGED arms.
  - N_LET cgstmt's per-kind `sz` cascade collapses to a single
    `if (lu->kind ∈ {ARRAY,SLICE,STR,STRUCT,TUPLE,TAGGED}) sz = lu->size`.
  - N_LET cgexpr's match-bind primitive sizing: `bsz = (int)bu->size`
    drops the TY_STR/TY_SLICE special-cases (same outcome — ty_str/
    ty_slice already have ->size set by type.c).
  - Three `sz == 16` / `let_emit_size(d->type) != 16` gates against the
    str slot width route through ty_str->size.

  Cap-offset sites (cgen.c:2440/1994/3206/5517 `delta = 16` for
  slice's .cap field-write) intentionally NOT touched: 16 there is the
  *offset of .cap inside a slice header*, structurally always 16
  regardless of str.size.  #1 doesn't move the slice layout.

Track C — lib/ user code:

  - lib/strings.freeall + appendstr, lib/shlex.freepartial + appendstr:
    the four `16u64` literals (per-str-element stride for rt_ensure and
    os.free) become `size(str): u64`.  Check-time fold via #42's
    intercept resolves to 16 today; #1 reroutes via the bumped tinfo.

After this commit, bumping ty_str to 24B for task #1 requires editing
exactly two places (cmd/wcc/type.c:64 ty_str.size, plus check.ww
primtypesize's "str" arm) for the SSoT to propagate.

Verification:
  - 131/131 tests pass.  994_w6c_ww + 995_self_rebuild byte-identity
    holds — each replacement evaluates to the same constant the
    literal had today, so cgen output is unchanged.
  - selfhost source's `size(str): u64` folds at check time (cstage
    cmd/wcc/check.c:907-960 for the C-bootstrap of selfhost; wwstage
    check.ww:898-942 for the rebuild path), no runtime call introduced.

selfhost/cmd/wcc/cgen.ww

@@ -965,7 +965,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 	let t: *node = d.lhs;
 	for (t != nil) {
 		if (t.kind == nkind.N_TPTR) { return 8; };
-		if (t.kind == nkind.N_TSLICE) { return 24; };
+		if (t.kind == nkind.N_TSLICE) { return tyslicesize(): i32; };
 		if (t.kind == nkind.N_TARRAY) {
 			let lenn: *node = t.rhs;
 			let elemn: *node = t.lhs;
@@ -987,7 +987,7 @@ fn letemitsize(c: *cgen, d: *node) i32 = {
 		if (letscalarprim(nm)) { return 8; };
 		let fsz: i32 = letfloatprim(nm);
 		if (fsz > 0) { return fsz; };
-		if (streq(nm, "str")) { return 16; };
+		if (streq(nm, "str")) { return primtypesize("str"): i32; };
 		let si: *structinfo = structlookup(c, nm);
 		if (si != nil) { return si.totsize; };
 		let next: *node = aliaslookup(c, nm);
@@ -1313,7 +1313,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitline("\"\n");
 					};
 				};
-				if (sz == 16 && !issg) {
+				if (sz == primtypesize("str"): i32 && !issg) {
 					let r: *node = d.rhs;
 					for (r != nil) {
 						if (r.kind != nkind.N_CAST) { break; };
@@ -1369,7 +1369,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 							emitsymname(c, nm);
 							emitline("(SB),\"");
 							let i: i32 = 0;
-							for (i < 16) {
+							let szstr: i32 = primtypesize("str"): i32;
+							for (i < szstr) {
 								emitdatawbyte(0u8);
 								i += 1;
 							};
@@ -1377,7 +1378,7 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						};
 					};
 				};
-				if (sz == 24 && !issg) {
+				if (sz == tyslicesize(): i32 && !issg) {
 					// Slice: zero-init only (no slice-literal
 					// syntax to honour). Any rhs other than
 					// `nil` is skipped → undefined symbol at
@@ -1399,7 +1400,8 @@ fn emitletdataw(c: *cgen, file: *node) void = {
 						emitsymname(c, nm);
 						emitline("(SB),\"");
 						let i: i32 = 0;
-						for (i < 24) {
+						let szsl: i32 = tyslicesize(): i32;
+						for (i < szsl) {
 							emitdatawbyte(0u8);
 							i += 1;
 						};