refactor(capture): rename hook point post_mlp -> post_block

[RhizoNymph]

What

Renames the activation-capture / steering hook point post_mlp -> post_block project-wide (136 files): the HookName Literal, the _HOOK_NAME_TO_ID map, SteeringHookPoint.POST_MLP -> POST_BLOCK (value "post_block") and its table-attr names, every validator (filesystem + graphsafe + steering protocols), the plan/manager/runner plumbing, all model files that register the hook, docs, examples, rust test fixtures, and tests.

Aligns the hook naming with PR #174 (which bundles this rename with other capture work); this PR is the rename portion in isolation.

Why

post_block more accurately names the residual-stream tap (after the full decoder block) and matches the naming used in #174's capture work, so the two don't diverge on the hook vocabulary.

Breaking change

This is a hard rename with no post_mlp alias. Any config/request using the old name must switch to post_block: capture hooks dicts, --capture-graphsafe-key <layer>:post_mlp, filesystem global_hooks, per-request SamplingParams.capture specs, steering module JSON ({"vectors": {"post_mlp": ...}}), and the OpenAI/serve steering APIs.

Note

The model RMSNorm attributes post_mlp_norm / post_mlp_layernorm are intentionally left unchanged — they are layernorm module/weight names unrelated to the capture/steering hook, and renaming them would risk breaking checkpoint weight-name matching.

Sibling PRs

Touches scheduler.py/manager.py, so it will conflict with the capture_wait PR and the already-merged capture PRs — resolve additively at merge.