[WIP] Support long-context and MTP prefix-cache hits

[grimoire]

Summary

This PR is a follow-up to the prefix-cache refactor in #4618.

It enables prefix-cache reuse in two cases that were previously rolled back or disabled:

• allow prefix-cache hits to resume long-context chunked prefill from the matched prefix instead of rolling back when the remaining suffix still needs chunking;
• enable prefix caching for Spec/MTP with one-block overlap recompute, so the target model recomputes the hidden-state bridge needed by the draft/MTP path;
• keep matched-but-recomputed overlap blocks private/writable during trie allocation, avoiding writes into shared cached KV blocks;
• handle SSM prefix-cache restore through exact ready checkpoints, including sparse checkpoint cases where the private recompute span may be larger than one block;
• add regressions for scheduler rollback, chunk flags, cached-token accounting, MTP overlap matching/allocation, VLM boundary expansion, and SSM checkpoint restore.