selfhost+test: single-source-of-truth @tagscr scratch reservation (#38)

Closes STATUS latent #1: @tagscr shared 24B reservation across the
four tagged-scratch sites (cgreturn, pushargsrev, cgindex
tagged-elem, pointer-rooted struct-field tagged write). Any fn that
needed >24B (e.g. slice-in-tagged-field 32B) silently overflowed
into the neighbor frame slot. Surfaced concretely as getopttest's
errortable wwstage exit 16 after #37 fixed the upstream gaps.

c.tagscrsz: i32 on the cgen struct is the single source of truth.
tagscrbump(c, need) in scanlocals raises the max across all 4
reservation sites and returns the frame delta. All emit sites
(cgreturn / pushargsrev / cgindex / cgwidentaggedstore pointer-
rooted) read c.tagscrsz instead of hardcoded 24. Mirrors the existing
cgwidentaggedstore precedent; @tagbase keeps its 8B scanseenmark
dedup (always 8B, correct).

Unmasked latent bug (now fixed): scanlocals's pointer-rooted struct-
field tagged-write detection uses localfindnode(c, base.str) to
resolve the *struct base. For `fn fill(h: *holder)`, h's scan-time
stub from scanseenmark had tnode=nil, so the @tagscr reservation
never fired. Pre-#38 the hardcoded 24B masked this; #38's correctly-
sized slot exposed it. cgfn's param scan loop now sets
c.locals.tnode = scanp.lhs after scanseenmark so localfindnode
resolves param types at scan time.

Test 714 (tagged_return_scratch): 4 rows × 2 stages = 8 fixtures.
Direct adjacency repro; match-arm field-by-field read; **mixed-
sizes-one-fn** (16B pushargsrev widen + 32B cgreturn widen in the
same body — pins the lockstep invariant that a sibling site can't
undersize the shared slot); call-site struct-payload widen. Row 3
specifically would regress if a future refactor ever forgets to
route an emit site through c.tagscrsz.

982 getopt_run green through both stages (was the original surface);
995 self_rebuild byte-id holds.

selfhost/cmd/wcc/cgenexpr.ww

@@ -3451,7 +3451,7 @@ fn cgassign(c: *cgen, n: *node) void = {
 					if (istaggedtype(c, elemtn)) {
 						let slot_sz: i32 = slotsize(c, elemtn);
 						let scroff: i32 = localadd(c, "@tagscr",
-						    24, nil);
+						    c.tagscrsz, nil);
 						// Pre-zero scratch (matches push helper).
 						emitline("\tXORQ\tAX, AX\n");
 						let zz: i32 = 0;