Every section banner dies (103 -> 0) across test/lang, the observer
suites, the C carriers, and the five comment-heavy corpus fixtures;
banner provenance (#N cites, carrier numbers, repair-cluster labels)
folded into headers or adjacent WHY comments. Narration deleted; row
provenance, ref cites, divergence pins, and layout contracts kept
(fwd-ref decl-order guards and bootstrap-gate corpus rationale
restored where the sweep over-cut). Comment-only proven: all 3742
wwbuild workdir .s byte-identical before/after; test-commit and
test-byteid (161 lang + 1399 data, 0 pinned-divergent) green.
The in-cap aggregate-receive materialise emitted a single narrow tail MOV that fell to MOVB for a 3/5/6/7-byte sub-8 tail, storing one byte while the scratch->dest copy read the full tail from uninitialised scratch — silently dropping members at the C2c whole-element arm (arr[i]=mk()) and loud-stopping at the #11 field arm. The scratch slot is ceil-8 padded (local_alloc/localadd round to 8) and the copy reads only tsz bytes, so flipping the tail default MOVB->MOVQ stores the full register harmlessly into the slot's own pad (in-bounds for in-cap <=24B); 1/2/4-byte tails stay byte-identical. Both stages symmetric. Removes the now-redundant #11 sub-8-tail loud-stop (keeps the float #165 and over-cap #234 loud-stops). The same narrow-tail materialise recurs at 6 other cstage sites (task #14).
Retires the obsolete idx_dot_aggret_subtail_loud //ww:error fixture (both stages now compile the case) and converts it to a positive cstage run-test; the struct-field shape is byte-id-divergent only via the pre-existing #9 frame-size bug, so the value pin uses array-field shapes. Value-asserting, reddens under each stage's independent revert.
