JIT: session_start lowering for self-host spine (ext/standard/session_start.php)

[PurHur]

Problem

ext/standard/session_start.php is in the self-host spine bundle (compiler_lib_spine_smoke requires it) but docs/capabilities.md lists session_start — JIT: no.

VM compliance is green (#64, test/compliance/cases/stdlib/session.phpt). Spine link either stubs the call (SelfHostBuiltinPolicy) or fails when real lowering is enabled — blocking native spine growth alongside #1880-class string JIT fixes.

Goal

session_start() (and minimal $_SESSION read/write used by spine/tests) lowers in JIT so spine units that touch session do not require ExternalMethod stubs.

Scope (phase 1)

• JIT stub or native lowering for session_start returning bool
• Session superglobal slot in JIT context (mirror VM session table)
• test/compliance/cases/stdlib/session.phpt passes under bin/jit.php
• Update php script/capability-matrix.php → docs/capabilities.md

Out of scope v1: session_write_close, custom handlers, file-based session GC parity with Zend.

Implementation hints

Layer	Files	Notes
VM reference	ext/standard/session_start.php, lib/VM.php	Copy semantics
JIT builtin	lib/JIT/Builtin/ or ext/standard/JitSession*.php	Follow header() / cookie patterns (#1184)
Policy	lib/JIT/SelfHostBuiltinPolicy.php	Flip session_start from stub when green
Spine	test/selfhost/compiler_lib_spine_smoke/main.php	Already requires session unit

Acceptance criteria

php bin/jit.php test/compliance/cases/stdlib/session.phpt
./script/ci-local.sh --filter SessionBuiltin

Docker:

docker run --rm -v "$(pwd):/compiler" -w /compiler php-compiler:22.04-dev \
  vendor/bin/phpunit --filter SessionBuiltin

Dependencies

• Stdlib: session_start() and $_SESSION (file or memory driver) #64 ✅ VM
• JIT: Self-host spine link — Struct::getElementType on __string__ pointer params #1880 (spine string JIT) — link order / spine smoke
• Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 self-host · CI: Default BOOTSTRAP_LIB_SPINE_VM_SMOKE_GATE=1 in ci-local (#1846 follow-up) #1867 spine CI gates
• Stdlib: floatval, boolval, gettype(null) and expanded PHPT specs #5-SessionsWeb example (consumer doc only)

Verification

Local/Docker only — #394 GHA disabled.

Links

• docs/stdlib-jit-audit.md · ROADMAP 🗺️ ROADMAP: Self-host + web-capable PHP subset (living) #78 Phase 2 Stdlib
• Related example: 005-SessionsWeb (when filed)

[PurHur]

Dependency fix: spine string/pointer JIT blocker is #1880 (not #1879).