compat: emit stock PHP ini defaults to match shivammathur/setup-php@v2

[phramz]

Context

PR #77 lands a PR-time compat-diff gate against a pinned v2 baseline. The canonical cell (noble/x86_64/8.4) currently goes red because our phpup install returns empty for ~12 ini keys per fixture that v2 emits via stock upstream defaults. See the sticky compat-report comment on #77 for the exact per-fixture list.

These are pre-existing gaps in internal/compose, not regressions in #77. PR #77 deliberately did NOT add blanket allowlist kind: ignore entries to mask them.

Gaps surfaced by the gate

Stock PHP defaults (v2 ships these via Ondrej's php.ini-production):

• expose_php = 1
• log_errors = 1
• max_input_time = -1
• session.save_handler = files
• session.gc_maxlifetime = 1440

OPcache defaults (v2's configs/ini/jit.ini fragment — see compat-matrix.md §2.3):

• opcache.enable = 1
• opcache.enable_cli = 0
• opcache.memory_consumption = 128
• opcache.revalidate_freq = 2
• opcache.validate_timestamps = 1
• opcache.jit = 1235
• opcache.jit_buffer_size = 256M (x86_64) / 128M (aarch64, per compat-matrix §2.4)

Xdebug defaults under coverage: xdebug (v2's configs/ini/xdebug.ini, §2.2):

• xdebug.mode = coverage
• xdebug.start_with_request = default

Scope suggestion

Likely a compose-layer change in internal/compose to ship the stock php.ini-production as the base SAPI ini, plus an opcache-loaded gate that overlays the jit.ini fragment and an xdebug-loaded gate that overlays xdebug.ini.

Acceptance

• Canonical cell noble/x86_64/8.4 in ci.yml::pipeline goes green without any allowlist suppression
• compat-golden-refresh.yml weekly run continues to show zero drift
• Documented in docs/compat-matrix.md if any subset is intentionally NOT replicated (with reason:)