Fix the sign of @inline bigint constants

[cknitt]

Bigint_utils.parse_bigint returns whether a literal is positive, but Ast_external_mk.inline_bigint bound that flag as negative, so every @inline bigint constant was stored with its sign inverted.

Translation passed the flag straight into Lambda's Const_bigint, whose flag means positive, which inverted it back. So generated JavaScript was correct, but signatures and error messages printed the wrong sign:

Values do not match:
  let x: bigint
is not included in
  @inline(-12n) let x: bigint   // declared as @inline(12n)

This renames the field to positive, matching Bigint_utils.parse_bigint, Asttypes.Const_bigint, Lambda.Const_bigint and Js_op.bigint_lit, which all record whether a bigint is positive. The flag now passes through the frontend and translation unchanged.

Found while looking into structural @inline constants (#8624).

Tests

• tests/tests/src/inline_const.res: a functor whose parameter declares @inline(12n) and @inline(-5n). A functor parameter is currently the only way to reach codegen with a bigint inline constant, since @inline let x = 12n isn't supported in implementations yet. This pins that the JS output keeps 12n / -5n.
• super_errors/inline_bigint_signature_mismatch.res: pins the printed sign of both constants in a signature mismatch.

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commit: 3b2c6f1

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.81%. Comparing base (1cc1da7) to head (3b2c6f1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8732      +/-   ##
==========================================
+ Coverage   79.79%   79.81%   +0.02%     
==========================================
  Files         464      464              
  Lines       63134    63134              
==========================================
+ Hits        50377    50391      +14     
+ Misses      12757    12743      -14     

Files with missing lines	Coverage Δ
compiler/frontend/ast_external_mk.ml	100.00% <100.00%> (+13.33%)	⬆️
compiler/ml/external_ffi_types.ml	95.00% <ø> (ø)
compiler/ml/translcore.ml	86.34% <100.00%> (+0.16%)	⬆️
compiler/syntax/src/res_outcome_printer.ml	63.36% <100.00%> (+1.04%)	⬆️

... and 3 files with indirect coverage changes

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[cknitt]

@codex review

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-07T06:40:12.707783Z	3b2c6f1	Manual request

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Codex Review: Didn't find any major issues. Another round soon, please!

Reviewed commit: 3b2c6f1492

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