fix: resolve pool-shutdown hang + reload SIGSEGV (#176) via ABI v0.24.0; pack http_request_t flags

[EdmondDantes]

Summary

Two long-standing flaky failures in the pool-shutdown / hot-reload tests were
runtime bugs fixed upstream in php-async, not server-repo bugs — the CI
box was simply running a PHP built one day before the fixes landed. This PR
records that finding and lands one unrelated struct-packing win from the review
backlog.

1. Root-caused two "known issues" → both resolved by ABI v0.24.0

• Pool graceful-shutdown teardown hang (core/055–057): confirmed via gdb
  on a live hang — the main thread blocks in php_module_shutdown → libuv_reactor_quiesce joining worker threads that never exit, because the
  bootloader's persistent coroutine is never cancelled on retire. Fixed by
  php-async 937bcfe (sync-worker graceful-shutdown backstop).
• Intermittent reload SIGSEGV (#176): cross-thread run_time_cache UAF from
  a shared nested-closure dynamic_func_defs. Fixed by php-async 0cbdfe8
  (deep-copy nested closures per worker, PR #180).

Both were verified here after rebuilding + reinstalling PHP to ABI v0.24.0:
055/056/057 PASS, and a synthetic #176 stress harness ran 30/30 clean
(≈50 % SIGSEGV before). The two docs/ISSUE_* files are updated to RESOLVED
with the full investigation trail. No server code change was needed for
either — the environment just needs php ≥ v0.24.0.

2. http_request_t: pack bool flags into a bit cluster (368 → 360 B)

The 10 per-request bool flags were 1 byte each and, with the neighbouring
1-byte scalars, rounded the struct tail up to a full 8-byte line. Convert them
to bool:1 bitfields and park the cluster in the 7-byte padding hole that
already sat after trace_flags. Named bitfields keep read/write ergonomics
(and 0/1 conversion) identical, so there are no use-site edits. A request
is single-owner (handed off by ownership, never mutated concurrently), so
non-atomic bit RMW is safe.

Testing

Full server phpt suite against the rebuilt v0.24.0 stack, all green:

suite	result	suite	result
core	57 pass (2 skip)	h3	51/51
grpc	15/15	static	23/23
h1	26/26	compression	12/12
h2	29/29	sendfile	5/5
reactor_pool	9/9	tls	15/15

Plus: 055/056/057 PASS, #176 stress harness 30/30 clean.

[github-actions]

Coverage

Total lines: 81.23% → 80.84% (-0.39 pp)

File	Baseline	Current	Δ	Touched
src/core/http_connection_tls.c	73.39%	73.67%	+0.28 pp
src/core/http_protocol_handlers.c	86.25%	80.00%	-6.25 pp
src/core/http_protocol_strategy.h	66.67%	33.33%	-33.33 pp
src/core/worker_dispatch.c	71.78%	62.37%	-9.41 pp
src/core/worker_registry.c	65.12%	61.63%	-3.49 pp
src/http3/http3_callbacks.c	81.30%	80.62%	-0.68 pp
src/http3/http3_dispatch.c	78.41%	78.12%	-0.28 pp
src/http3/http3_listener.c	76.72%	72.74%	-3.98 pp
src/http3/http3_steer.c	100.00%	87.50%	-12.50 pp
src/http3/http3_stream.c	96.39%	93.98%	-2.41 pp