cgen: B5-c2 #73 graduation — the 6 tripwired field gates chase, tripwire deleted

Close-by-construction replacing containment — the designed graduation
path from F1-c1's commit body. The 6 fld_alias_tripwire call sites
(indexed-elem field store/read, heap struct-lit field fill, tuple-elem
read, ptr-chain field read, static struct-lit emit) chase their fu
through type_chase_named; the 6 tripwire calls AND the helper itself
(incl. its :447 peel) are DELETED. Raw `->under` in cgen.c 69→62.
#73 CLOSES.

TRAIN INVARIANT holds: cs-only; w6c_ww/ww_ww bit-identical to the
bcd948d baseline md5s (28ad889042bad8006f1997cbcec94805 /
4e9ca8741f19e1f68219ff799a5e5a14). ZERO new corpus movers vs the c1
tip (kb*/kna corpus + five mains byte-NEUTRAL — the chased gates only
fire on 2+-level alias aggregate fields, none in corpus).

Gate-arm graduations (pre = loud "#73" fatal on cs, ww ok/0; post =
0/0 BYTE-ID): slice/str/tagged arms at the indexed STORE gate
(g73_idxstore/g73_strfield/g73_tagfield), slice arm at the indexed
READ / ptr-chain READ / tuple-elem READ gates (g73_idxread/g73_ptrread/
g73_tupread), nested-struct arm at the static emit gate
(g73_static_struct). The existing 944_alias_accept_run tripwire bound
row graduates K_BUILDERR_CS → K_RUN.

Two arms cannot pin the full 0/0 byte-id cell — documented, not silent:
  - heap fill (g73_heapfill, COMPILE-only pin): bare /tmp programs
    never link malloc (both stages, pre-existing infra) and ww's
    deref-field READ carries the #24-kin field(SB) leak; the chased
    FILL bytes verified byte-id by hand against the plain (non-alias)
    control — divergence shape identical, all of it in the pre-existing
    read sites.
  - str-field static emit (g73_static_str): the #73 fatal gave way to
    the pre-existing #129 A.2 foldability loud on cs — now both-loud
    twin texts (fill0 class), pinned as K_BUILDERR.
  - struct-copy arm: NO runnable repro reaches these gates — indexed
    whole-struct field reads take the (already chased, byte-id) address
    spine, and tuple-of-struct louds upstream on BOTH stages (#54-kin
    "aggregate init from unhandled rhs shape"). The arm's only carrier
    was the heap gate, covered by the compile pin above.

944_alias_cgen_b5_run grows 16→25 rows (65 checks); all 944-family
suites green; sizelint 0.

test/wcc/944_alias_accept_run.c

@@ -460,12 +460,11 @@ static const struct row rows[] = {
 	  "	if (w[2] != 900) { return 2; };\n"
 	  "	return 0;\n"
 	  "};\n", 0, K_RUN, NULL },	/* graduated: #60 (F2a batch 1) */
-	/* #73 tripwire pin: the UNPROBED single-peel field gates
+	/* #73 GRADUATED (B5-c2): the tripwired single-peel field gates
 	 * (indexed-elem store/read, ptr-chain read, heap fill, tuple-elem
-	 * read, static emit) hard-error on a 2+-level alias over an
-	 * aggregate base instead of emitting a word0 tail. ww accepts and
-	 * runs these — cs-only loud until #73's chase. 1-level controls
-	 * verified per gate (/tmp/revF1 t*c probes). */
+	 * read, static emit) now chase the field type — the loud bound
+	 * flipped to working 0/0 byte-id. Per-gate-arm rows live in
+	 * 944_alias_cgen_b5_run. */
 	{ "slicefield_idx_tripwire_73",
 	  "package main;\n"
 	  "type sl = []int;\n"
@@ -477,8 +476,7 @@ static const struct row rows[] = {
 	  "	xs[0].s = a[0:3];\n"
 	  "	if (xs[0].s.len != 3) { return 1; };\n"
 	  "	return 0;\n"
-	  "};\n", 0, K_BUILDERR_CS,
-	  "unswept single-peel gate (#73)" },
+	  "};\n", 0, K_RUN, NULL },	/* graduated: #73 (B5-c2) */
 	/* Values >255 break the esz=1 prefix-luck; the write-then-read
 	 * pins the IMULQ stride on both the read and write spines. */
 	{ "idx_2level",