Compile in-process on OCaml domains in OCaml rewatch

[cknitt]

Stacked on #8780.

This PR moves compilation into the OCaml rewatch process. Instead of starting bsc for every parse and compile, rescript links the compiler and runs requests on a bounded pool of OCaml 5 domains. That removes process start-up and pipe handling, which make the subprocess port about 6% slower than Rust on clean builds. It also opens the door to sharing compiler state between requests later.

Changes

• The compiler's process-wide mutable state is made request-scoped. Clflags, Js_config, warnings, environment and CRC caches, predefined types, Ident stamps, printers, diagnostics and backend caches move into domain-local state. Almost all of it lives in Request_local keys: every request starts with fresh values, as a new bsc process would, and the previous values are restored afterwards. Only Ident stamps, the request context, output capture and the order-sensitive predefined types keep explicit handling. The driver moves from rescript_compiler_main.ml to rescript_compiler_driver.ml, so bsc and rewatch share it.
• Each request's compiler output goes to request-owned buffers, written in text mode so that Windows output keeps bsc's CRLF line endings.
• Rewatch schedules independent parse and compile requests on worker domains. By default that's at most 8, and one fewer than the CPU count; REWATCH_COMPILER_DOMAINS overrides it. Artifact publication stays on the scheduling domain. PPXs and hooks still run as external processes.
• rescript format also runs on the embedded compiler. PPXs, js-post-build, --after-build and the lock-owner check are the only remaining subprocesses. RESCRIPT_BSC_EXE now only affects rescript-rust, and CONTRIBUTING.md says how to test a local compiler instead.
• lib/bs/compiler-info.json still records the bsc.exe shipped next to rescript.exe. The VS Code extension reads bsc_path to find bsc.exe and the other platform binaries.

Known issues before this can leave draft

• An earlier domain build once produced a differing output that was never explained. Review since found and fixed shared or leaked state:

  • Js_analyzer's free-variable accumulator was shared between domains, so Js_shake could drop a live function definition; this reproduced once in CI;
  • Parmatch.Conv's name counter, Ctype's abbreviation memo and levels, PPX cookies and the parser hooks for error messages.

  An audit of top-level mutable records, visitors and tables found no other live case. Regression tests cover the Js_analyzer race and the leaked Ctype memo.

• No ThreadSanitizer run over a parallel build yet.

• bsc now buffers its output until exit. Diagnostics no longer stream, and warnings already emitted are lost if an internal compiler error escapes.

• The focused suite's slow-compiler cases no longer apply; signal handling is covered through a slow js-post-build hook instead.

Tests

tests/rewatch_ounit_tests/compiler_driver_tests.ml checks that each kind of request-scoped state is isolated between consecutive and concurrent requests. rewatch-ocaml/bench compares clean-build artifacts across domain counts; it does not run in CI.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 80.22042% with 341 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.74%. Comparing base (03d9bf4) to head (0d1e9f7).
⚠️ Report is 6 commits behind head on rewatch-ocaml-port.

Files with missing lines	Patch %	Lines
compiler/bsc/rescript_compiler_driver.ml	73.16%	73 Missing ⚠️
compiler/ml/ctype.ml	81.99%	47 Missing ⚠️
compiler/ml/printtyp.ml	60.34%	46 Missing ⚠️
compiler/gentype/debug.ml	51.21%	20 Missing ⚠️
compiler/ml/env.ml	84.34%	18 Missing ⚠️
compiler/ext/compiler_request_output.ml	77.77%	10 Missing ⚠️
compiler/ml/typecore.ml	91.73%	10 Missing ⚠️
compiler/core/js_implementation.ml	79.06%	9 Missing ⚠️
compiler/ml/cmi_format.ml	50.00%	8 Missing ⚠️
compiler/ml/ast_mapper.ml	53.33%	7 Missing ⚠️
... and 50 more

Additional details and impacted files

@@                  Coverage Diff                   @@
##           rewatch-ocaml-port    #8781      +/-   ##
======================================================
+ Coverage               80.66%   80.74%   +0.07%     
======================================================
  Files                     463      468       +5     
  Lines                   62792    63575     +783     
======================================================
+ Hits                    50651    51331     +680     
- Misses                  12141    12244     +103     

Files with missing lines	Coverage Δ
analysis/src/module_resolution.ml	18.18% <100.00%> (ø)
compiler/common/js_config.ml	81.81% <100.00%> (+6.81%)	⬆️
compiler/core/bs_conditional_initial.ml	100.00% <100.00%> (ø)
compiler/core/cmd_ppx_apply.ml	77.96% <100.00%> (+0.77%)	⬆️
compiler/core/config_util.ml	93.75% <100.00%> (-6.25%)	⬇️
compiler/core/ir_diagnostics.ml	92.59% <100.00%> (ø)
compiler/core/js_analyzer.ml	82.91% <100.00%> (-0.32%)	⬇️
compiler/core/js_cmj_format.ml	94.54% <100.00%> (+0.20%)	⬆️
compiler/core/js_dump.ml	92.84% <100.00%> (+<0.01%)	⬆️
compiler/core/js_dump_program.ml	98.33% <100.00%> (ø)
... and 103 more

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commit: 2fff740