Bootstrap: fix duplicate SqrtJitHelper spine require (#14968)

[PurHur]

Summary

• Removed duplicate SqrtJitHelper.php require_once in compiler_lib_spine_smoke/main.php (spine was 3727 vs 3726 inventory after Fix DatePeriod inline DateTime ctor args losing backing state (#15124) #15133–php-in-php: asin() JIT via AsinJitHelper PHP (#15130) #15135)
• Synced M2 spine footnotes to 3726/3726 across README and bootstrap docs
• Updated prelinked/bootstrap-gen0/.m3_compiler_lib_sidecar.sha to match spine entry SHA-1

Closes #14968

Verification

php script/bootstrap-inventory.php --check
# OK 3726/3726

php script/bootstrap-spine-count.php
# bootstrap-spine-count: 3726/3726

php script/check-selfhost-spine-count-sync.php
# check-selfhost-spine-count-sync: OK (spine 3726/3726 from compiler_lib_spine_smoke)

php script/check-selfhost-spine-deferred-sync.php
# check-selfhost-spine-deferred-sync: OK (0 deferred: (none); spine 3726/3726)

php script/check-selfhost-spine-sidecar-sync.php
# check-selfhost-spine-sidecar-sync: OK (stamp matches spine entry SHA-1 325179a78231448901a9a1eb8ccb6c84076b1d12)

php script/check-wave3-roadmap-sync.php
# check-wave3-roadmap-sync: OK (language open 0, stdlib open 0, spine 3726/3726)

make bootstrap-selfhost-link
# bootstrap-selfhost-link: OK .../build/selfhost

Lane B context

No eligible non-stdlib implementation issues in lane B (n % 3 == 1); bootstrap fallback fixed check-selfhost-spine-deferred-sync red gate on master.

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