[YouHengfei]

Purpose

Refs #32 and #111 (M9 track A).

Types the PD control-plane protocol without changing the wire format or behavior:

• New pd/protocol.py: CommandKind (coordinator→worker), EventKind (worker→coordinator), PendingPhase, and ProfileAction as str enums. Members compare equal to their string values; messages over the Pipe remain the same plain dicts.
• Pending.phase becomes a typed PendingPhase instead of a raw string (finding F6 in docs/pd_walkthrough.md, currently in [Doc][M9] PD and KV transfer walkthrough #64).
• Session-scoped profiling commands (profile_*) are dispatched through an explicit command→action map instead of string prefix manipulation, keeping them separate from transfer-scoped messages as [RFC] PyTorch profiling interface with coordinated PD capture and per-rank artifacts #71 requires.

Organized as two commits: the first adds CPU characterization tests that pin the current protocol behavior (state machine phases, credit accounting, cancel/cancel_safe ordering, command field sets, profile-reply isolation); the second performs the typing. The tests pass unmodified before and after.

Test Plan

• New CPU coordinator tests in tests/test_pd.py (mocked processes/pipes).
• pytest tests/test_pd.py tests/test_profiling.py (CPU).
• Full CPU suite.
• pytest tests/test_pd.py --run-gpu with real multi-process NIXL on 2× H100.

Test Result

• tests/test_pd.py + tests/test_profiling.py (CPU): 38 passed, 15 skipped (GPU).
• Full CPU suite: 564 passed, 170 skipped.
• GPU (CUDA_VISIBLE_DEVICES=2,1, UCX_TLS=tcp,sm,self,cuda_copy,cuda_ipc): 32 passed, 1 skipped (the 4-GPU multi-worker test). Covers Ouro + Nanbeige handoff, last_exited/shared/CUDA Graph combinations, cancel/reuse at every protocol boundary, prefix-reuse transfer reduction, and EOS after handoff.
• ruff check / ruff format --check: clean.

[YouHengfei]

This PR is intentionally split into two commits:

First commit adds CPU characterization tests that pin the current protocol behavior (phase transitions, credit accounting, cancel/cancel_safe ordering, command field sets) against the unmodified code.
Second commit performs the typing. The tests pass unchanged on both sides, which is the behavior-preservation evidence.

Scope note: This PR types message kinds and transfer phases only. Payloads remain plain dicts on purpose — the Pipe wire format is unchanged, and converting payloads to dataclasses would touch every send site and bloat the diff. Typed payloads (e.g. reserve/commit/ack records in pd/protocol.py) are a natural follow-up once this lands, after the M3/M4/M5 contract discussions in #111 settle the field ownership.
GPU coverage: The multi-process NIXL suite passes on 2× H100 (32 passed, 1 skipped for the 4-GPU case), so the str-enum messages survive pickle/Pipe round trips on the real path.