fix: correct RWKV7 prefill state contract

[inureyes]

Summary

• Disable chunked prefill for RWKV7 because multi-token no-ID forwards intentionally reset model-owned recurrent state.
• Add a loaded one-layer regression proving independent no-ID prompts reset and stable sequence-ID chunks retain equivalent recurrent state.
• Restore the RWKV7 step-output shape contract by removing the singleton matrix-product axis before stacking timesteps.

Root cause found during validation

The loaded regression exposed an existing RWKV7 forward failure. Each recurrent step computed state @ r[..., None] as [B, H, D, 1] and stacked those tensors without removing the final singleton axis. This produced [B, T, H, D, 1], while the following per-head normalization expects [B, T, H, D] and weights shaped [H, D]. The pre-fix GPU run aborted with a native broadcast error between (2,4) and (1,1,2,4,1).

The fix squeezes only the matrix product's final axis, yielding the documented per-step [B, H, D] output and stacked [B, T, H, D] sequence tensor. Recurrent state remains [B, H, D, D].

Test plan

• Pre-fix focused GPU run reproduced the RWKV7 output-shape failure.
• GPU-locked rwkv7_ focused suite after the shape fix: 5 passed, including the loaded-model contract test in 0.64 seconds.
• Root and surgery CUDA test-fast rebuild after the shape fix: passed in 2 minutes 22 seconds.
• Contract gate: all pre-Python stages passed initially; after lowering competing build priority, both timing-sensitive guard tests passed, followed by all 123 Python tooling and 49 WebUI tests. No code or test was weakened for the retry.
• git diff --check

Dependency refresh

The validation above was recorded before merging main commit 4b0c4519 from PR #2298. The current branch merged that MLX pin and overlay update at 9b05507b without changing src/models/rwkv7.rs (SHA-256 fa18e8e5e33f4af42fe7c7c8574e55db8578f90724d3a70d0cad4a7cf10adea7). Focused runtime revalidation on the new pin is pending; the checked results remain historical evidence for the reviewed model source.

Closes #2286