Preserve snapshots until retention expiry

[SaladDay]

Capacity pressure could shorten a suspended allocation’s checkpoint retention and discard its memory to admit another Session. Preserve snapshots until the configured retention deadline instead. A full retained pool keeps new requests pending with their original deadline; active-pressure suspension remains available when it can free usable capacity. Natural expiry still requires confirmed resource cleanup before capacity is reused.

Removes the early-retention path and obsolete test branches; updates the owning English and Chinese lifecycle documentation. No cross-node protocol or placement changes.

Validation: 445 deployment, placement, PostgreSQL and lifecycle tests/subtests passed with a real isolated database, with zero skips; server build and relevant vet passed; three translation checks passed. Regression coverage includes ten Sessions sharing three retained slots, repeated lifecycle observation under pressure, unchanged input deadlines and natural expiry cleanup. Independent full-diff review found no remaining issues.

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