Re-key the append() lowering from BP-displacement assumptions onto a
resolver-provided header PLACE (task #15, the add_thread hard-blocker;
cgplaceaddr's third consumer after C1/C1.25). One mirrored choke-point,
two failure modes: cstage 0-defaulted sn_off for any non-ident target,
so 0(BP)/8(BP) became the "slice header" and rt_ensure corrupted the
CALLER frame (SIGSEGV); wwstage cgappend silently emitted nothing
(gate-blind cs!=ww).
cg_append_grow/cg_append_slot (mirror cgappendgrow/cgappendslot) factor
the 5 grow + 5 slot header-access sites. Ident-local targets keep the
legacy BP-disp emission byte-identical (probed across all 9 existing
source shapes, before/after .s). Non-ident targets resolve once through
cgplaceaddr and spill the header address to an @apphdrscr slot:
rt_ensure may realloc .ptr but never moves the header, so the slot
stays valid; every access reloads from it. The slot is allocated fresh
per append SITE, not cached per fn: a nested append-through-pointer
inside a value expression (match-yield arm) spills its own resolve, and
a shared slot would hand the outer grow/slot reloads the inner target's
header — silent cross-slice corruption (pinned by the reentrant_value
row). Indirect mode keys esz/element-kind/load-op off the
checker-stamped target tinfo (no declared tnode behind `*p`; the
#209/#211 discipline). Unwired target places die LOUD "#15: append()
target place unsupported (rule-7)" on BOTH stages — the
silent-corruption class is closed by construction.
The FA4/#35 boundary is unchanged: non-ident spread SOURCES stay loud
(pinned by a reject row). Surfaced pre-existing checker divergence
filed as task #34 (wwstage rejects global slice-lit let).
test/wcc/806: 14 runtime rows (element kinds x target shapes, spread,
narrow-signed spread load, cap-crossing realloc loop with branched
callee + caller-frame sentinels, deref-spine target, nested-append
reentrancy, direct-arm neutrality pin) + 2 exact-text reject rows,
both drivers + per-row cs==ww asm byte-id.
Hojun-Cho
b630a7cf20