Cosmos3-edge PR 2: KV retention policy and partial release for windowed generation

[merceod]

Ports the windowing module and the KV release contract from #198 onto the resource-pool engine (resources/kv/manager.py), so windowed autoregressive video can bound the committed context of a long rollout.

What changed

• RetentionPolicy(context_budget, protected_prefix) on a CacheStream (the existing stub, filled in) and KVManager.set_retention(request_id, policy, label=None). commit applies the policy right after a stream's span is counted. The oldest unprotected whole pages go back to the arena, stored_len drops by the freed count, and generation bumps so the dense backend re-gathers its prefix. This happens inside the commit, under the lock and before commit_done, so the pre-plan of the next step already sees the compacted stream and no admitted plan addresses freed pages.
• KVManager.protect_prefix / release_oldest for explicit between-step drivers. Refused under an admitted step or while offloaded.
• mstar/engine/windowing.py with WindowSchedule (pure window arithmetic, from Sliding Window Attention (AR video gen) + KV cache lifecycle management #198) and WindowedKVSession.bind(prefix), which installs the schedule's context horizon as the stream's retention.

Why the release moved into the commit

#198 released pages between steps on the old allocator. With the pool's speculative pre-planning there is no safe moment between steps anymore (the next step is admitted and planned while the current one runs), so the release became a stream-level policy applied at commit. Streaming ASR or duplex speech models can use the same policy.

Tests

test/modular/test_kv_release.py (compaction, page floor, boundary and tail pages, admitted-step refusal, later plans over the compacted stream, page conservation, commit-time retention with sub-page carry-over, validation, reset) and test/modular/test_windowing.py. ruff and the full test/modular pass. The Cosmos3 kv-mode rollout that drives this end to end is verified on H100 in #297.