CI: default BOOTSTRAP_M3_HELLOWORLD_STRICT_GATE=1 in ci-local (#1866)

[PurHur]

Summary

• Flip BOOTSTRAP_M3_HELLOWORLD_STRICT_GATE default from 0 to 1 in script/ci-defaults.env (M3 HelloWorld strict native emit runs in ci-local.sh LLVM tail by default).
• Align ci-common.sh fallback, docs/local-ci-matrix.md, docs/bootstrap-selfhost.md, and phpc doctor --selfhost presenter text.
• Update CiScriptsTest assertions for default-on + opt-out =0.

Closes #1866

php-src / Zend reference

CI wiring only — probe behavior unchanged (script/bootstrap-selfhost-helloworld-probe.sh, issue #1526 / #1493).

Verification

./script/docker-exec.sh -- bash -lc \
  'php vendor/bin/phpunit --filter "testCiLocalHonorsBootstrapM3StrictGate|testLocalCiMatrixDocumentsBootstrapM3StrictGate" test/unit/CiScriptsTest.php'

PHPUnit 9.6.21 by Sebastian Bergmann and contributors.
..                                                                  2 / 2 (100%)
OK (2 tests, 13 assertions)

Issue repro (probe green per #1866 harness note — strict gate now default in full CI):

./script/docker-exec.sh -- bash -lc 'make bootstrap-selfhost-helloworld'
# Expected on standard harness: bootstrap-selfhost-helloworld-probe: OK emit_path=native

Opt-out documented: BOOTSTRAP_M3_HELLOWORLD_STRICT_GATE=0 ./script/ci-local.sh

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