M3 parser unit probe: CFG parse front-end native link (#2409)

[PurHur]

Summary

• Adds test/selfhost/parser_unit_probe/ with compiler_minimal-scale bundle (lib/Runtime.php, lib/Compiler.php, Lint stack), a class/method/if fixture.php, and parser_unit_probe_parse_smoke() for Zend PHPCfg parse coverage.
• Adds script/bootstrap-selfhost-parser-unit-probe.sh and make bootstrap-selfhost-parser-unit-probe (prints parser_unit_probe bundle OK at build/selfhost-parser-unit-probe).
• Documents probe in docs/bootstrap-selfhost.md / docs/self-host-target.md; BootstrapParserUnitProbeTest covers lint, native link, and parse smoke.

Follow-ups (separate issues): BOOTSTRAP_PARSER_UNIT_PROBE_GATE in ci-local (#2417), north-star3-verify step 5 (#2418), default-on flip (#2419).

Closes #2409

Test plan

• ./script/docker-exec.sh -- bash -lc 'composer install --ignore-platform-reqs && ./script/apply-patches.sh && source script/php-env.sh && php bin/compile.php -l test/selfhost/parser_unit_probe/main.php'
• ./script/docker-exec.sh -- bash -lc '… && ./script/bootstrap-selfhost-parser-unit-probe.sh'
• vendor/bin/phpunit test/unit/BootstrapParserUnitProbeTest.php

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