feat(patch): multi-hook sweeps + source-run lifecycle

[RhizoNymph]

Stacked on #235 (base feat/patch-sweep-spans).

Adds first-class multi-hook activation-patching sweeps and a source-run lifecycle so one-call sweeps stop leaking (and polluting the LRU with) fresh uuid runs.

Multi-hook sweeps

• New hooks: list[str] | None on PatchSweepRequest. When given (non-empty, each injectable — validated, 400 otherwise) the (layers x positions) grid runs at every hook; hooks wins over the single hook field. The corrupt baseline and noise floor are computed once and shared across hooks; only patched cells fan out per hook.
• Response gains hook_grids: list[HookGrid] (one {hook, grid, argmax} per hook). Top-level grid/hook/argmax mirror the first hook, so single-hook clients keep working; positions/layers/clean/corrupt/noise_floor/alignment/skipped stay shared (skipped cells carry their hook). hooks=["post_block"] matches hook="post_block" bar the extra hook_grids entry.
• Client sweep_layers_positions(..., server_side=True, hooks=[..]) returns dict[str, SweepResult] keyed by hook; passing hooks on the per-cell path raises ValueError.

Hook-complete auto-capture

• _auto_capture_clean now taps all three injectable hooks (pre_attn, post_attn, post_block) at the swept layers in one forward, so a kept auto-captured run is reusable for hook-comparison follow-up sweeps at a different hook.

Source-run lifecycle

• Auto-drop: after a sweep that auto-captured completes, the fresh run is dropped (recent uuid runs sit at the LRU MRU end and would evict older deliberate captures first). keep_source: bool = False retains it; a pre-existing run is never dropped. Drop failures are log-warn.
• DELETE /v1/patch_source/{run_id} -> collective_rpc("drop_patch_source_run") (new gpu_worker RPC mirroring lease_patch_source_runs), unioned across ranks: 200 {"dropped": true} or 404. An explicit owner drop succeeds even if the run is leased (store drop_run clears the lease); LRU eviction still never touches a leased run.
• Both drop paths invalidate the admission-side _PatchSourceCache so a dropped run is not reported as still-existing (which would skip auto-capture and 400 the next sweep).
• Client PatchStudy.drop_run wired to the DELETE route (returns bool); one-call keep_source passthrough.

Conflicts

Expected merge conflicts with sibling branch feat/patch-sweep-stream (SSE streaming) in api_router.py's fan-out section, protocol.py, and docs/features/activation_patching.md.