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]

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Reviewed commit: e45b706218

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[chatgpt-codex-connector]

Add behavioral coverage for labeled-arrow unification

Add a lambda/end-to-end regression that typechecks or calls through equivalent ~x: int => string and (~x: int) => string annotations. The changed snapshots only verify the parsed arity metadata, so the user-facing claim that these spellings now unify could regress in later compiler layers while every added test continues to pass; repository guidance explicitly requires syntax, lambda, and end-to-end coverage for compiler changes.

AGENTS.md reference: AGENTS.md:L39-L41

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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 75.58%. Comparing base (9043bbb) to head (e45b706).

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                           75.59%   75.58%   -0.01%     
==================================================================
  Files                                 476      476              
  Lines                               62759    62745      -14     
==================================================================
- Hits                                47443    47428      -15     
- Misses                              15316    15317       +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	91.48% <85.71%> (-0.05%)	⬇️

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commit: e45b706