feat(#3038): JIT unit probe inventory compile_driver emit path

[PurHur]

Summary

• Add test/selfhost/jit_unit_probe/compile_driver.php — inventory-scale M3 emit driver for the JIT unit probe (parity with compiler_unit_probe/compile_driver.php, Self-host M3: inventory emit driver for unit probes (retire *_m3_emit_native_entry.php) #2879).
• Update bootstrap-selfhost-jit-unit-probe.sh to prefer inventory compile_driver when BOOTSTRAP_M3_JIT_UNIT_PROBE_STRICT=1 (default) or BOOTSTRAP_M3_USE_INVENTORY_EMIT_DRIVER=1; bisect with BOOTSTRAP_M3_USE_INVENTORY_EMIT_DRIVER=0 to force jit_unit_probe_m3_emit_native_entry.php.
• Makefile strict target sets BOOTSTRAP_M3_USE_INVENTORY_EMIT_DRIVER=1 explicitly.

Advances #2879 / #3032 (retire *_m3_emit_native_entry.php for JIT probe).

Test plan

make bootstrap-selfhost-jit-unit-probe-strict
# → emit_path=native, inventory compile_driver (no emit-helper TU, #2879/#3038)

BOOTSTRAP_M3_USE_INVENTORY_EMIT_DRIVER=0 make bootstrap-selfhost-jit-unit-probe-strict
# → bisect: legacy jit_unit_probe_m3_emit_native_entry.php path

vendor/bin/phpunit --filter BootstrapJitUnitProbeTest

Verified (Docker):

bootstrap-selfhost-jit-unit-probe: OK emit_path=native ... inventory compile_driver (#2879/#3038)
jit-unit-probe-aot stdout: jit unit probe compile OK
PHPUnit BootstrapJitUnitProbeTest: 14/14 OK

Closes #3038

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