Add exclusive compiler phase tracing for OCaml rewatch

[fhammerschmidt]

Summary

• Add an opt-in REWATCH_TYPECHECK_TRACE that records exclusive compiler phase timings, allocations, and request-level GC data across OCaml rewatch workers.
• Add a trace analyzer and a benchmark runner for comparing traced and plain builds.
• Document the measured type-checking, dependency-loading, and artifact-writing costs, including the limits of the measurements.

Stack

Depends on #8671. This PR adds the detailed compiler trace and benchmark analysis on top of that performance work.

Validation

• make checkformat
• make test

[codecov]

Codecov Report

❌ Patch coverage is 69.32271% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.69%. Comparing base (80eb959) to head (26ec8df).

Files with missing lines	Patch %	Lines
compiler/ext/compiler_phase_trace.ml	18.75%	39 Missing ⚠️
compiler/bsc/rescript_compiler_driver.ml	70.68%	17 Missing ⚠️
compiler/ml/cmi_format.ml	78.37%	8 Missing ⚠️
compiler/ml/env.ml	82.35%	6 Missing ⚠️
compiler/core/bs_cmi_load.ml	78.94%	4 Missing ⚠️
compiler/core/res_compmisc.ml	83.33%	2 Missing ⚠️
compiler/ml/typemod.ml	97.29%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                    @@
##           faster-rewatch-ocaml    #8672      +/-   ##
========================================================
- Coverage                 78.73%   78.69%   -0.04%     
========================================================
  Files                       480      481       +1     
  Lines                     65110    65183      +73     
========================================================
+ Hits                      51264    51296      +32     
- Misses                    13846    13887      +41     

Files with missing lines	Coverage Δ
compiler/core/js_implementation.ml	84.55% <100.00%> (+0.12%)	⬆️
compiler/ml/typemod.ml	73.88% <97.29%> (+0.16%)	⬆️
compiler/core/res_compmisc.ml	86.95% <83.33%> (+0.59%)	⬆️
compiler/core/bs_cmi_load.ml	78.94% <78.94%> (+1.16%)	⬆️
compiler/ml/env.ml	77.18% <82.35%> (+0.09%)	⬆️
compiler/ml/cmi_format.ml	79.31% <78.37%> (+1.31%)	⬆️
compiler/bsc/rescript_compiler_driver.ml	75.49% <70.68%> (+0.07%)	⬆️
compiler/ext/compiler_phase_trace.ml	18.75% <18.75%> (ø)

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[fhammerschmidt]

Superseded by consolidated PR #8680, which preserves this work as one commit in the combined review branch.