Add K3 three-stage RL path: proxy checkpoint tooling, RL data prep, and fused-layout checkpoint cleanup

[ISEEKYAN]

What

The implementation side of the K3 three-stage RL path, cut fresh from main with no experiment scaffolding.

• lite/checkpoint.py: the fused attention input projection is no longer concatenated with self-computed offsets, and per-projection semantics are no longer derived by matching substrings of the parameter name. The layout is declared and the offsets are derived from that declaration.
• lite/fused_layout.py: K3 now owns the minimal row-axis checkpoint layout declarations it needs, removing the dependency on the unmerged MLite fused-weights module.
• lite/protocol.py: K3 owns decoder-layer selection for R3 router replay; this folds the former Fix K3 R3 router discovery with current MLite #21 into this PR.
• lite/model.py, config.py: the declaration surface (per-projection name, head count, head dim, order) and the config plumbing behind it.
• primitive/router.py: explicit FP32 gating contract for the shared router.
• tools/build_proxy_checkpoint.py, tools/prepare_gsm8k_rl_from_sft.py: reproducible construction of the proxy checkpoint and of the RL dataset from the SFT split.

Why

Weight synchronisation into the inference engine was producing a deterministic, silent corruption of one segment of the fused seven-way projection: the same element index and the same bad-element count reproduced byte-for-byte across independent runs, while the source-side tensor was entirely finite. Hand-computed fusion offsets and name-substring parsing were the mechanism; declaring the layout removes the class of bug rather than one instance of it.

Keeping the K3-specific layout declaration in this package also removes the import closure that previously required an unmerged MLite PR.

Validation

• K3 checkpoint layout contracts: 5 passed.
• K3 R3 replay-root contract: 1 passed.
• Ruff format check, Ruff check, compileall, and git diff --check passed.
• A full local run against the available MLite checkout produced 61 passed, 1 skipped, and 6 failures in pre-existing QAT/MXFP4 paths because that checkout lacks megatron.lite.primitive.quantization.qat and differs in its MXFP4 export contract. These paths were not changed by this update.
• GPU validation of the end-to-end path is not included here and is tracked separately.

Note

Opened as a draft: the GPU re-verification of the synchronisation path has not run yet.