Replace CPPO browser branches with platform modules

[cristianoc]

Replace BROWSER conditionals with Dune-selected ordinary modules. The browser profile selects playground implementations while every other profile selects native implementations, keeping dev and release builds functionally identical and documenting the convention where it is applied.

Keep the platform boundary narrow: native builds use C primitives and artifact persistence, while the playground uses JavaScript-compatible primitives and no-op persistence. Keep shared Cmt_format behavior in an ordinary module and use generated source rules instead of virtual libraries, which trigger a Dune internal error with the compiler's unwrapped libraries and break @check and ocaml-lsp.

Remove CPPO from compiler sources that no longer need preprocessing, remove the unused hash_small_int test hook, and retain CPPO only for genuine type-specialization templates. Update architecture guidance and the error-variant catalog for the new source layout.

Signed-off-by: Cristiano Calcagno cristianoc@users.noreply.github.com

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[codecov]

Codecov Report

❌ Patch coverage is 87.71499% with 50 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.78%. Comparing base (9f26fa6) to head (b0c5f42).

Files with missing lines	Patch %	Lines
compiler/ext/ext_string.ml	83.10%	37 Missing ⚠️
compiler/ml/cmt_format_common.ml	68.18%	7 Missing ⚠️
compiler/core/lam_compile_main.ml	96.62%	5 Missing ⚠️
compiler/ext/ext_sys.ml	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8541      +/-   ##
==========================================
+ Coverage   74.70%   74.78%   +0.08%     
==========================================
  Files         460      466       +6     
  Lines       62389    62821     +432     
==========================================
+ Hits        46608    46983     +375     
- Misses      15781    15838      +57     

Files with missing lines	Coverage Δ
compiler/core/lam_module_ident.ml	86.66% <ø> (ø)
compiler/ext/hash_set_ident_mask.ml	96.42% <100.00%> (+0.06%)	⬆️
compiler/ext/js_reserved_map.ml	88.88% <100.00%> (ø)
compiler/ml/cmt_format.ml	100.00% <100.00%> (ø)
tests/ounit_tests/ounit_hash_stubs_test.ml	68.42% <100.00%> (-3.01%)	⬇️
compiler/ext/ext_sys.ml	0.00% <0.00%> (ø)
compiler/core/lam_compile_main.ml	96.62% <96.62%> (ø)
compiler/ml/cmt_format_common.ml	78.18% <68.18%> (ø)
compiler/ext/ext_string.ml	83.10% <83.10%> (ø)

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commit: b0c5f42

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

Reviewed commit: b0c5f42419

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[cknitt]

Great to get rid of CPPO! Platform-specific modules are a much better mechanism. 🎉