Record written arrow arity before external lowering

[cristianoc]

Summary

• record an arrow type's written parameter count directly in the parsetree, including phantom @as(...) _ parameters on externals
• remove the parser-side decrement and matching printer compensation
• give bare labeled arrow types the same arity as their parenthesized spelling

Rationale

External lowering already removes phantom parameters and rebuilds the function type with its effective call arity. Encoding that lowered arity in the parser made the surface parsetree context-dependent and required the printer to reverse the adjustment. Keeping the written arity makes the parser representation consistent while preserving generated JavaScript.

Bare labeled arrow types previously printed like their parenthesized equivalent but carried different arity metadata and therefore did not unify with it.

Validation

• make test-syntax
• make test
• parser snapshots cover the bare labeled spelling
• existing external fixtures retain byte-identical generated JavaScript

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: e45b706218

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[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.83%. Comparing base (cbd02f1) to head (f138a72).

Files with missing lines	Patch %	Lines
compiler/syntax/src/res_core.ml	85.71%	1 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           codex/harden-parsetree0-bridge    #8563      +/-   ##
==================================================================
- Coverage                           75.84%   75.83%   -0.01%     
==================================================================
  Files                                 476      476              
  Lines                               62729    62715      -14     
==================================================================
- Hits                                47575    47560      -15     
- Misses                              15154    15155       +1     

Files with missing lines	Coverage Δ
compiler/syntax/src/res_parsetree_viewer.ml	90.48% <ø> (-0.13%)	⬇️
compiler/syntax/src/res_core.ml	91.53% <85.71%> (-0.05%)	⬇️

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commit: f138a72

[cknitt]

Nice to get rid of the arity workaround for @as(json...)! 🎉

You may want to address the Codex comment before merging.