Gen-1+ #6: Bootstrap SDK release artifacts (versioned gen-0 tarball)

[PurHur]

Problem

Gen-0 seeds live only inside the git repo (prelinked/bootstrap-gen0/). External contributors need a Bootstrap SDK release separate from user phpc v1.x.

Acceptance

• GitHub Release asset: php-compiler-bootstrap-${TAG}-linux-x86_64.tar.gz containing gen-0 driver, vendor .o, manifest, sidecar stamp

• phpc bootstrap init --sdk-url or env PHP_COMPILER_BOOTSTRAP_SDK

• Version tied to spine count + sidecar SHA-1

• North-star epic: Epic: Self-host critical path — compiler compiles itself (M3→M5) #1492 · blocks gen-1+ only dev workflow · see docs/bootstrap-dev-workflow.md

[PurHur]

claim: cursor-agent-lane-c — spine sidecar stamp drift after #16964/#16965 spine entries; refreshing stamp + footnotes

[PurHur]

claim: cursor-agent-lane-c — implementing --sdk-url / PHP_COMPILER_BOOTSTRAP_SDK fetch path (#15602)

[PurHur]

Merged via #17652

Verification:

./script/phpunit.sh --filter BootstrapHonestCompileLibTest
# OK (14 tests, 59 assertions)

make bootstrap-sdk-pack TAG=test-sdk
# → build/php-compiler-bootstrap-test-sdk-linux-x86_64.tar.gz

phpc bootstrap init --sdk-url file://…/php-compiler-bootstrap-*.tar.gz
# or PHP_COMPILER_BOOTSTRAP_SDK=<release-url> phpc bootstrap init

Acceptance: release tarball naming (TAG-linux-x86_64), fetch path (--sdk-url / env), spine SHA-1 in pack output.