feat(server): add prefix cache to paged concurrency (qwen AR)

[Graffioh]

Summary

• Enable copied in-memory prefix checkpoints for Qwen continuous batching with paged attention. Full-prefill and disk snapshots remain disabled in paged mode.
• Keep token matching, boundary selection, pinning, LRU, and hit accounting in PrefixCache while Qwen owns checkpoint payloads, page-table restore, and recurrent state.
• Introduce an opaque checkpoint/ticket protocol and a move-only capture transaction so the scheduler never manipulates backend snapshot storage.
• Gather and scatter logical K/V rows through fresh sequence-owned pages, copy the matching recurrent slab, coalesce consecutive physical blocks, and publish replacements atomically.

Correctness review

The pre-draft review fixed these lifecycle hazards:

• stale restore cleanup now happens before busy or failed admission returns;
• unsupported engines use cold admission without deleting a valid checkpoint;
• malformed or mismatched events can abort only their own capture ticket;
• fatal step failures discard the pending destination instead of preserving potentially mutated state;
• failed replacement leaves the incumbent checkpoint intact;
• cache hits and MRU promotion are recorded only after a validated restore;
• exact-prompt checkpoints cannot mask a shorter restorable prefix when next-token logits are unavailable;
• tensor type, contiguity, extents, vector topology, recurrent slab shape, and per-layer tensor pairs are validated before destination writes.

Verification

• HIP Release build of dflash_server
• test_server_unit: 418 passed
• test_seq_slot_manager: 186 checks
• test_parallel_prefix_txn: 21 checks
• test_seq_engine_contract: 19 checks
• test_recurrent_snapshot: 2 passed
• git diff --check

Draft gates

• Measure capture and restore pause time plus snapshot memory at realistic long-prefix sizes while decode traffic is live.
• Run real Qwen GPU parity for a partial final page and a forced mid-message boundary, restoring into non-identity fresh pages.
• Decide the copied-checkpoint byte budget before broad production enablement.

This PR targets main as the AR ownership/mechanism base. Open speculative concurrency PRs #642 and #654 overlap in SeqEngine, Qwen, scheduler, and target-graph files; they should consume this opaque prefix-store boundary during their rebase rather than duplicate snapshot ownership.

[Graffioh]

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[cubic-dev-ai]

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[cubic-dev-ai]

All reported issues were addressed across 25 files

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[cubic-dev-ai]

All reported issues were addressed across 26 files

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[cubic-dev-ai]

All reported issues were addressed across 10 files (changes from recent commits).

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