Drop dead isinstance check; split Scheduler._submit_ready_job

[ajslater]

Summary

Two warnings flagged by `make typecheck complexity`:

picopt/config/__init__.py

Confuse 2.2.0 already types `config.get(MappingTemplate)` as `AttrDict[str, object]`, so the defensive `isinstance(ad, AttrDict)` check after it was statically always-true:

```
warning: Unnecessary isinstance call; "AttrDict[str, object]" is always an instance of "AttrDict[Unknown, Unknown]" (reportUnnecessaryIsInstance)
```

Dropped the check and the now-unused `AttrDict` import.

picopt/walk/scheduler.py

`Scheduler._submit_ready_job` was rank C in radon. Split into three smaller pieces, each rank A/B:

• `_drop_cancelled_ready_job` (staticmethod) — counter cleanup for a leaf of a cancelled subtree (`UnpackJob`/`RepackJob` skip; only leaf jobs decrement the parent counter, since the cancel walk already decremented for node-owned jobs).
• `_track_submitted_job` — record an in-flight future under the right map and update node state.
• `_submit_ready_job` — orchestrate: pop, cancelled-skip, submit, track.

Behavior unchanged.

Verification

• `make typecheck` — `0 errors, 0 warnings, 0 notes`
• `make complexity` — no findings (was rank C on `_submit_ready_job`)
• `uv run ruff check picopt/` — clean
• `uv run pytest` — 150 passed, 6 skipped (the previously-broken `test_timestamp_parents` now passes thanks to the merged treestamps 4.0.2 fix)

🤖 Generated with Claude Code