Add MapDataReplayState coordinator class

[ltiao]

Summary:
The experiment replay system (MapDataReplayMetric, MapDataReplayRunner, replay_experiment) is hardcoded for single-objective optimization, blocking multi-objective early stopping. MapDataReplayMetric conflates data serving with progression state, so multiple metrics cannot share a coherent timeline. This diff series extracts shared state into a MapDataReplayState coordinator.

This diff (1/3) adds the MapDataReplayState class -- a shared state coordinator that manages normalized cursor-based progression across multiple metrics and trials. Uses a global min/max MAP_KEY to preserve cross-metric timing alignment. Serves original MAP_KEY values; downstream ESS normalizes independently via _maybe_normalize_map_key.

Existing MapDataReplayMetric and helpers are unchanged in this diff.

Differential Revision: D98741817

[meta-codesync]

@ltiao has exported this pull request. If you are a Meta employee, you can view the originating Diff in D98741817.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.68421% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.41%. Comparing base (227243c) to head (2f64057).

Files with missing lines	Patch %	Lines
ax/metrics/map_replay.py	95.65%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5137    +/-   ##
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  Coverage   96.40%   96.41%            
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  Files         613      613            
  Lines       68191    68342   +151     
========================================
+ Hits        65740    65889   +149     
- Misses       2451     2453     +2     

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