[KV Connector][NIXL] Add hybrid KDA target-state transport

[Sy0307]

TL;DR

Adds fail-closed NIXL Push transport for hybrid MLA + KDA by transferring only target convolution history and base recurrent state, then initializing RecoverSSM-only records locally. It also adds homogeneous PP-stage pairing and DCP-aware rank/completion mapping so replicated MLA and TP-sharded KDA state cannot be confused.

Purpose

Add an explicit, fail-closed NIXL Push transport contract for hybrid MLA + KDA deployments where the producer and RecoverSSM consumer use different physical KDA page layouts.

The connector now:

• transfers only the target convolution history and base recurrent state instead of copying an opaque KDA page;
• initializes RecoverSSM-only verify, correction, and key/gate records locally on the consumer;
• rejects ordinary speculative scratch blocks and connectors that do not declare this capability;
• carries DCP metadata through the push handshake, maps replicated MLA slices separately from TP-sharded KDA state, and waits for every overlapping DCP producer;
• supports homogeneous TP/PP deployments by pairing matching pipeline stages and rejects mismatched hybrid PP layouts;
• includes DSpark prompt-context transport policy in the compatibility handshake so incompatible peers cannot silently connect.

This is an early-review Draft. The intended first topology is homogeneous P/D TP, PP, and DCP with NixlPushConnector, RecoverSSM, and DSpark context KV. It does not claim heterogeneous PP or NIXL Pull support.

Related tracking: #40017. This complements the Mooncake-specific work in #54875 but uses NIXL Push and preserves speculative decoding when compatible DSpark context pages are present.

Test Plan

• CPU unit tests for no-spec to RecoverSSM target-state conversion with block4/block7 and DCP1/DCP4 cases.
• Unit tests for homogeneous RecoverSSM materialization and rejection of ordinary speculative scratch blocks.
• NIXL descriptor tests ensuring attention block IDs cannot address KDA staging regions.
• TP/DCP mapping and push completion-count tests for overlapping MLA ranks and TP-sharded KDA state.
• Homogeneous PP stage-selection and mismatched-HMA-PP fail-closed tests.
• Follow-up GPU E2E validation on homogeneous TP8 x PP2 x DCP4 P/D nodes before marking ready for review.

Test Result

• ruff check: passed for all changed files.
• ruff format --check: passed for all changed files.
• typos: passed for all changed files.
• Python bytecode compilation: passed.
• git diff --check: passed.
• PyTorch/pytest and GPU E2E tests were not run in this macOS worktree because PyTorch and pytest are unavailable. This PR remains Draft until targeted Linux/GPU validation is attached.

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, including scope and related tracking.
• The test plan and exact validation categories.
• The currently available test results and explicit missing GPU coverage.
• No user-facing documentation update is required while this remains an experimental Draft.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @Sy0307.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork