feat(M88): Framework-Level Inference Hooks

[hlin99]

Summary

Implements framework-level inference hooks for capturing intermediate states during inference, enabling root-cause analysis of PD divergence by comparing intermediate representations.

Changes

• inference_hooks.py: Core module with:
  • InferenceHook protocol: on_prefill, on_kv_transfer, on_decode_step
  • HookCapture, StageComparison, TraceResult dataclasses
  • MockInferenceHook for testing with configurable noise scale
  • compare_captures(): field-by-field comparison (max/mean abs diff, cosine similarity)
  • run_trace(): orchestrates hook-based comparison across pipeline stages
  • format_trace(): rich terminal output with per-stage comparison table
• CLI: xpyd-acc trace subcommand with --baseline, --target, --prompt, --hooks, --mock, --json, --threshold
• 34 tests covering hook protocol, captures, comparison, trace, formatting, JSON export, CLI integration

Closes #188

[hlin99-Review-Bot]

✅ Approved (hlin99-Review-Bot)

Idea Value: High — framework-level inference hooks fill a critical gap for root-cause analysis of PD divergence. The hook protocol + mock pattern enables testing without live endpoints.

Code Quality: Clean.

• Well-defined protocol (InferenceHook) with clear hook points
• Solid dataclass hierarchy with proper serialization
• compare_captures() handles edge cases (shape mismatch, None fields)
• 34 tests with good coverage (protocol, captures, comparison, trace, formatting, JSON, CLI)
• CI all green (lint + tests on 3.10/3.11/3.12)
• docs/iterations/current.md updated

LGTM.

[hlin99-Review-BotX]

✅ Approved (hlin99-Review-BotX)

Idea Value: High — inference hooks are essential for pinpointing where PD divergence originates. The protocol-based design with clear hook points (prefill/kv_transfer/decode_step) is the right abstraction.

Code Quality: Clean.

• InferenceHook protocol is well-defined with runtime_checkable
• compare_captures() handles shape mismatches and None fields correctly
• _cosine_sim properly handles zero-norm edge case
• run_trace() cleanly orchestrates the comparison pipeline
• Mock hook with configurable noise enables deterministic testing
• 34 tests, CI green on 3.10/3.11/3.12
• docs/iterations/current.md updated

Second approval — should auto-merge. LGTM.