Add string literal regression coverage

[cknitt]

This is the first PR in the String Theory series and the base of a two-PR stack addressing #8602.

It adds characterization and regression coverage for string literals before changing their compiler representation. Some tests intentionally capture existing incorrect behavior and document the corresponding JavaScript semantics.

Coverage includes:

• escape sequences, Unicode, surrogate pairs, and line continuations
• equivalent string spellings in pattern matching
• JavaScript UTF-16 string semantics
• ordinary, backquoted, tagged, JSON, and raw literals
• the frozen AST v0/PPX bridge
• parsing, diagnostics, constant folding, and generated JavaScript

The second PR in the stack, #8606, normalizes the representation and updates the expectations for the bugs fixed there.

Testing

• make test
• make test-syntax
• make test-gentype
• make test-analysis

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-02T11:05:01.724402Z	0e82fe3	Manual request

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💡 Codex Review

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Reviewed commit: 59ed00eacc

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.00%. Comparing base (ebc61dd) to head (0e82fe3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8605      +/-   ##
==========================================
+ Coverage   76.97%   77.00%   +0.03%     
==========================================
  Files         467      467              
  Lines       62452    62461       +9     
==========================================
+ Hits        48070    48100      +30     
+ Misses      14382    14361      -21     

Files with missing lines	Coverage Δ
tests/ounit_tests/ounit_ast_mapper0_tests.ml	87.87% <100.00%> (+1.21%)	⬆️

... and 8 files with indirect coverage changes

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[cknitt]

@codex review

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commit: 0e82fe3

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 97fb7655a4

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[cknitt]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Already looking forward to the next diff.

Reviewed commit: 0e82fe38c9

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