perf(advisory): share AnySecurityAdvisory via Rc

PHP's SecurityAdvisoryPoolFilter stores advisory *object references* in
$securityRemovedVersions, and PoolOptimizer::applyRemovalsToPool hands
that array to the new Pool by copy-on-write. Porting AnySecurityAdvisory
as a value type turned both of those into deep copies.

Measured on `require laravel/framework` (offline, warm cache): 113
distinct advisories were duplicated into 314,309 copies of ~1.08 KiB,
retaining 331.9 MiB in the filter loop and another 331.9 MiB when
apply_removals_to_pool cloned the whole map.

3.54s -> 2.62s (-26%), peak RSS 975 MB -> 343 MB, which matches the upper
bound measured by ablation. Composer runs the same workload in 1.49s.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

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