[PHP] fix: preserve process-shared state across fork and exec

[brandonpayton]

Purpose

Preserve authoritative process, descriptor, and shared-memory state across
fork() and exec() so PHP-FPM and other multi-process POSIX software use the
normal Kandelo platform path.

This replaces the stale split branch's eight unique commits on an altered
socket integration base with nineteen reviewed, append-only repair commits on
the exact Batch 2 aggregate tip (e45019127). It intentionally does not replay
the altered socket base wholesale.

Reviewed result

• Distinguishes normal exit codes from signal termination and preserves Rust's
  zombie/wait authority through host worker lifecycle transitions.
• Corrects mmap placement, rounded unmap geometry, PID allocation, and
  fallible exec preparation before destructive state changes.
• Commits exec in place against the authoritative process, preserving surviving
  OFDs and kernel-backed objects while closing CLOEXEC descriptors and resetting
  image-specific state.
• Shares AF_UNIX accept queues, anonymous/SysV and regular-file MAP_SHARED
  backings, and refcounted eventfd/timerfd/signalfd/memfd/procfs state across
  fork/spawn/exec at the correct ownership boundary.
• Coordinates direct fork from instrumented side modules with versioned,
  transactional table/archive replay and rollback.
• Preserves signal, epoll, thread-worker, and sleep state at process lifecycle
  boundaries. Host-originated shared signals select the exact eligible TID;
  ignored signals do not shorten sleeps, and no specialized host dispatch
  resumes guest code after signal death.
• Documents the supported process-sharing semantics and the remaining
  WebAssembly/host boundaries.
• Regenerates the ABI snapshot/bindings for additive kernel exports and declares
  the exit-signal query optional for compatible ABI-16 hosts.

Review changes

The review rebuilt the constituent rather than force-fitting its stale socket
ancestry. The nineteen commits repair:

1. wait status, mmap/unmap geometry, PID collision avoidance, and exec growth;
2. AF_UNIX fork sharing and in-place exec state preservation;
3. anonymous/SysV and regular-file shared mappings, including stable backend
   identity, dirty/version merging, direct-I/O coherence, partial unmaps,
   MAP_FIXED preflight, and exact lifecycle publication;
4. shared in-kernel descriptor backings and side-module fork coordination;
5. epoll, worker/thread registry, anonymous-mapping detach, and concurrent
   pthread sleep correctness;
6. ABI-16 optional-export metadata and inactive-coherence fast paths; and
7. exact-thread signal routing plus terminal post-dispatch boundaries and
   re-entrant address-space teardown guards.

Independent final audits found no remaining constituent blocker.

Review view

Accept into quarantined Batch 2 via rebase merge. Do not land this PR directly
to main. The repaired constituent is coherent on the current #871 tip and
keeps its changes linearly revertible, but Batch 2 still needs the single atomic
ABI 16→17 reconciliation for the two divergent ABI-16 epochs and rebuilt PHP /
opcache artifacts.

Known boundaries remain explicit:

• file MAP_SHARED coherence occurs at mediated syscall/lifecycle boundaries,
  not as immediate shared physical memory;
• cross-process futex synchronization is not implemented;
• OPFS backends without stable inode identity and shared memfd mappings return
  ENOTSUP;
• beyond-EOF mapped pages are clipped/zero-filled rather than raising SIGBUS;
• unmediated external host writers are not detected; and
• the side-module role marker remains optional only for ABI 16 and becomes
  mandatory with the aggregate ABI-17 bump.

Validation

All claimed commands ran through scripts/dev-shell.sh.

Evidence	Result
cargo test -p kandelo --target aarch64-apple-darwin --lib	1,066/1,066 passed at Rust/ABI tip 4987bae1d; no Rust/ABI/generated file changed afterward
cargo test -p fork-instrument --target aarch64-apple-darwin	183/183 passed
cargo test -p wasm-posix-shared --target aarch64-apple-darwin	19/19 passed
focused dump_abi tests	10/10 passed
bash scripts/check-abi-version.sh at 953d8afb9 vs e45019127	passed; one optional host export plus eight kernel exports are additive-compatible; ABI remains 16; snapshot/C/TS outputs synchronized
cd host && npx vitest run at 953d8afb9	108 files passed, 30 skipped; 940 tests passed, 2 expected failures, 166 skipped; zero failures
seven focused signal/process/mmap files	148 passed, 2 expected failures, 8 skipped
cd host && npm run build	ESM, CJS, and declaration bundles built
npx tsx examples/run-example.ts examples/mmap_shared_anonymous_fork.wasm	both coherence markers and PASS

Final nine-round microbenchmarks used identical benchmark artifacts on the
candidate and exact e45019127 base:

Host / suite	Base	Candidate 953d8afb9
Node pipe/write/read MB/s; getpid us	22.31 / 154.82 / 229.41 / 11.71	22.57 / 152.67 / 225.89 / 10.40
Node hello/fork/exec/clone/spawn ms	152.00 / 44.40 / 194.40 / 40.76 / 42.85	155.64 / 43.86 / 195.87 / 40.82 / 43.02
Chromium pipe/write/read MB/s; getpid us	90.91 / 200 / 250 / 11	83.33 / 166.67 / 250 / 12
Chromium hello/fork/clone/spawn ms	93.21 / 9 / 6 / 8	115.56 / 10 / 7 / 9

The Node medians are close. Chromium uses coarse buckets and its candidate
hello rounds ranged from 80.98 to 171.31 ms, so these data are not presented as
a performance-neutrality claim.

Per the locked Batch 2 policy, this constituent did not run the full
ready-to-merge libc/POSIX/PHP/PHPT/application gate, manual ./run.sh browser
verification, or the full PHP browser PHPT suite. Those run once on the exact
ABI-17 aggregate. The isolated PHP image is not final-runnable here because
the harness interrupt import is supplied by #757 and final package artifacts
must be rebuilt for ABI 17.

[github-actions]

Phase B-1 matrix build status — pr-756-staging

ABI v16. 63 built, 4 failed, 67 total.

Package	Arch	Status	Sha
libcurl	wasm32	built	a6f6534d
libcxx	wasm32	built	0d3c1977
libcxx	wasm64	built	3117bdac
libiconv	wasm32	built	1be46463
libiconv	wasm64	built	82d9fdf3
libpng	wasm32	built	697eac68
libxml2	wasm32	built	41fada20
libxml2	wasm64	built	b3752126
openssl	wasm32	built	96fbd5a6
openssl	wasm64	built	b53b8ff0
sqlite	wasm32	built	10a86a4f
sqlite	wasm64	built	e74c37db
zlib	wasm32	built	1b3d5c6f
zlib	wasm64	built	8470f519
bc	wasm32	built	645e81c1
bzip2	wasm32	built	85ec619a
coreutils	wasm32	built	d0c76ab8
curl	wasm32	built	3dfec8e7
dash	wasm32	built	04d2f6b8
diffutils	wasm32	built	c804830a
dinit	wasm32	built	e646ca69
fbdoom	wasm32	built	935a71b0
file	wasm32	built	b9e73f9f
findutils	wasm32	built	aeeb6ecc
gawk	wasm32	built	9c6d61e9
git	wasm32	built	ad9f9d06
grep	wasm32	built	c6aa7ff9
gzip	wasm32	built	6c492249
kandelo-sdk	wasm32	built	fa69e4fc
kernel	wasm32	failed	—
less	wasm32	built	87aa211f
lsof	wasm32	built	c88ab5c9
m4	wasm32	built	5847a852
make	wasm32	failed	—
mariadb	wasm32	built	d5d40cc7
mariadb	wasm64	built	8c75bf16
modeset	wasm32	built	e5967428
msmtpd	wasm32	built	cff25a9b
nano	wasm32	built	d850deac
ncurses	wasm32	built	9a77777f
netcat	wasm32	built	e8e76c42
nginx	wasm32	built	3742a74e
php	wasm32	built	6c215a1d
posix-utils-lite	wasm32	built	3ab77df9
sed	wasm32	built	ef08c754
spidermonkey	wasm32	built	d1b3e35c
tar	wasm32	built	7ac89a61
tcl	wasm32	built	1e64df4d
unzip	wasm32	built	3f650bbb
userspace	wasm32	built	4dcfe62b
vim	wasm32	built	9c5d375e
wget	wasm32	built	2b0aaa25
xz	wasm32	built	dba87aba
zip	wasm32	built	f1c66da0
zstd	wasm32	built	4381b467
bash	wasm32	built	479f8133
mariadb-test	wasm32	built	95e7de82
mariadb-vfs	wasm32	built	9cca8b82
mariadb-vfs	wasm64	built	97bcd722
nethack	wasm32	built	6198392e
node	wasm32	built	cfe2bfc0
spidermonkey-node	wasm32	built	eee81883
vim-browser-bundle	wasm32	built	cb17bc3a
nethack-browser-bundle	wasm32	built	0d6d8f1e
rootfs	wasm32	failed	—
shell	wasm32	failed	—
node-vfs	wasm32	built	f8a54c30

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