fork-instrument: replay pure scalar fork inputs

[brandonpayton]

Summary

Adds conservative pure scalar materialization to wasm-fork-instrument so replay-safe call arguments and condition-only nested carryovers are recomputed at the post-call/post-landing point instead of spilled through frame locals.

The whitelist is intentionally small: scalar constants, scalar local.get, and non-trapping scalar integer unary/binary ops. Calls, memory/table ops, globals, refs, div/rem, float ops, local.set/local.tee, and any stack-dependent suffix still fall back to the existing spill-local path.

PR #701 MRE Evidence

Measured with /tmp/kandelo-pr701-mre, Node v24.15.0, node --experimental-wasm-exnref, max cap 20000. Depth ties on this host, but declared locals show the frame-pressure reduction.

artifact	benchmark_walk locals	max_survived
uninstrumented baseline	4	9,959
pre-PR713 old artifact (chain.before.wasm)	8	9,959
PR #713 packed base (chain.pr713-packed.wasm)	6	9,959
this branch (chain.kd-pme.wasm)	4	9,959

Local-by-local: the PR #713 packed base had benchmark_walk locals (i32 i32 i32 i32 i32 i64). This branch has (i32 i32 i32 i64), matching the uninstrumented baseline. The two removed synthetic i32 locals are the recursive walk(depth - 1) argument spill and the eqz(depth) IfElse condition/carryover spill. Both are now replayed from pure scalar tails.

Verification

• cargo test -p fork-instrument --target aarch64-apple-darwin --test instrument call_with_ -- --nocapture — 3 passed, 0 failed
• cargo test -p fork-instrument --target aarch64-apple-darwin --test instrument call_indirect_replays_pure_table_index_arg -- --nocapture — 1 passed, 0 failed
• cargo test -p fork-instrument --target aarch64-apple-darwin --test switch_dispatch omits_frame_header_state_locals -- --nocapture — 4 passed, 0 failed
• cargo test -p fork-instrument --target aarch64-apple-darwin --test switch_dispatch pr701_shape_replays_pure_condition_and_recursive_arg -- --nocapture — 1 passed, 0 failed
• scripts/dev-shell.sh cargo test -p fork-instrument --target aarch64-apple-darwin — 171 passed, 0 failed
• scripts/dev-shell.sh bash scripts/check-abi-version.sh — ABI snapshot/header/TS bindings up to date; ABI_VERSION and snapshot consistent
• git diff --check — clean

Not run: full kernel/host/libc/posix gates. This change is isolated to the fork-instrument rewrite tool and documentation; no kernel ABI, host runtime, libc, syscall, package, or browser path changed.

[github-actions]

Phase B-1 matrix build status — pr-714-staging

ABI v15. 52 built, 0 failed, 52 total.

Package	Arch	Status	Sha
bc	wasm32	built	ed7ce7b8
bzip2	wasm32	built	892df627
coreutils	wasm32	built	6c6b2f4d
curl	wasm32	built	ead181eb
dash	wasm32	built	c629e5d8
diffutils	wasm32	built	456f5322
dinit	wasm32	built	b39f70a8
fbdoom	wasm32	built	d7ae7642
file	wasm32	built	02660c1a
findutils	wasm32	built	a6979448
gawk	wasm32	built	77f47cb4
git	wasm32	built	dff0aff7
grep	wasm32	built	d89c4145
gzip	wasm32	built	d3e9efc9
kandelo-sdk	wasm32	built	d9cb395c
kernel	wasm32	built	71628a17
less	wasm32	built	bf2ac5a0
lsof	wasm32	built	0712dd67
m4	wasm32	built	9846d848
make	wasm32	built	4b7f856d
mariadb	wasm32	built	99eb4106
mariadb	wasm64	built	6011c9d9
modeset	wasm32	built	2fa4678e
msmtpd	wasm32	built	0f5d3cd7
nano	wasm32	built	699fea5c
ncurses	wasm32	built	3b90d7f5
netcat	wasm32	built	3041ca64
nginx	wasm32	built	8d390dca
php	wasm32	built	7cdc999c
posix-utils-lite	wasm32	built	f108b0ac
sed	wasm32	built	371b7e6e
tar	wasm32	built	6858b329
tcl	wasm32	built	5b7a648e
unzip	wasm32	built	78615f88
userspace	wasm32	built	1cdedce6
vim	wasm32	built	e032afe9
wget	wasm32	built	fd11dcb9
xz	wasm32	built	013788d1
zip	wasm32	built	b9c1bdc7
zstd	wasm32	built	5218e086
bash	wasm32	built	693899e0
mariadb-test	wasm32	built	c027b38a
mariadb-vfs	wasm32	built	956f0ad1
mariadb-vfs	wasm64	built	e8842c11
nethack	wasm32	built	44186439
vim-browser-bundle	wasm32	built	526382ba
nethack-browser-bundle	wasm32	built	80fb1ada
rootfs	wasm32	built	23a0788f
shell	wasm32	built	d47bb129
lamp	wasm32	built	a399cb6c
node-vfs	wasm32	built	794aab2f
wordpress	wasm32	built	becde562

_{Auto-generated; replaced on each push. Raw data in the publish-status workflow artifact.}