Fix lost and unstable comments around callbacks

[cknitt]

Trailing comments in nested callbacks could move outward across successive formatting passes, and comments on labeled or optional callbacks could disappear. For example, formatting call(~fn=x => x /* keep */) dropped the comment.

Share the argument-span calculation between comment attachment and printing so both use the same location. An explicit fallback rule selects the regular argument layout for trailing comments and leading comments on labeled arguments, checking both the full argument and expression locations. Leading comments on unlabeled arguments retain the compact layout to preserve stability when parameter parentheses are removed. Force a line break for trailing callback line comments so they remain inside the argument list.

Share callback label printing and assemble the call document once while preserving the existing layout-specific break behavior.

Add regression coverage for labeled, optional, first, last, and nested callbacks with leading and trailing comments. The regression test checks comment text and order preservation as well as formatting stability at five print widths, reusing the parsed intermediate output.

Fixes #6976.

Validation:

• make test-syntax
• make test-syntax-roundtrip
• dune exec syntax_tests
• make checkformat
• Exploratory audit of 2,700 argument/comment/width combinations: no comment loss or formatting instability after the fixes.
• The final helper and test refactor introduces no snapshot changes.

Known limitation: a separate pre-existing layout instability remains at width 20 for a comment inside a braced callback body used as the first argument, such as call(x => { /* comment */ x }, value), including labeled variants.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-06T14:20:39.199668Z	8179a59	Manual request

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[chatgpt-codex-connector]

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Reviewed commit: 3ca9d2eda3

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.58%. Comparing base (0b4bd0a) to head (8179a59).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8627   +/-   ##
=======================================
  Coverage   77.58%   77.58%           
=======================================
  Files         475      475           
  Lines       64051    64062   +11     
=======================================
+ Hits        49693    49704   +11     
  Misses      14358    14358           

Files with missing lines	Coverage Δ
compiler/syntax/src/res_comments_table.ml	89.06% <100.00%> (-0.01%)	⬇️
compiler/syntax/src/res_parsetree_viewer.ml	91.42% <100.00%> (+0.07%)	⬆️
compiler/syntax/src/res_printer.ml	93.28% <100.00%> (+0.02%)	⬆️

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commit: 8179a59

[github-actions]

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[cknitt]

@codex review

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Reviewed commit: faabd790b0

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[cknitt]

@codex review

[chatgpt-codex-connector]

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

Reviewed commit: 8179a59795

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