Fix unstable formatting of trailing callback comments

[cknitt]

Trailing comments in nested callbacks could move outward across successive formatting passes. Use the regular argument layout when arguments have trailing comments so formatting is stable after the first pass.

Add regression coverage for nested callbacks, first and last callback arguments, and line and block comments, including stability checks at five print widths.

Fixes #6976.

Validation:

• make test-syntax
• make test-syntax-roundtrip
• dune exec syntax_tests
• make checkformat

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-04T19:59:20.542567Z	3ca9d2e	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 3ca9d2eda3

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
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• Comment "@codex review".

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[chatgpt-codex-connector]

Link the changelog entry to the implementing PR

For this user-facing formatter fix, the new entry ends with the issue URL (/issues/6976), but repository guidance requires entries in the current Unreleased section to end with the implementing PR link. Replace this URL with the PR URL so the changelog follows the required attribution format.

AGENTS.md reference: AGENTS.md:L150-L150

Useful? React with 👍 / 👎.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.32%. Comparing base (cf94640) to head (c66b8ec).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8627   +/-   ##
=======================================
  Coverage   77.32%   77.32%           
=======================================
  Files         467      467           
  Lines       63313    63317    +4     
=======================================
+ Hits        48957    48961    +4     
  Misses      14356    14356           

Files with missing lines	Coverage Δ
compiler/syntax/src/res_printer.ml	92.96% <100.00%> (+<0.01%)	⬆️

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commit: c66b8ec