fix(js-parser): export FILE_BOUNDARY at module level for cross-parser parity

[gadievron]

The JavaScript unit generator declared FILE_BOUNDARY as a function-local
const inside _assembleEnhancedCode and exported only { UnitGenerator }, so
require(...).FILE_BOUNDARY was undefined. Every sibling parser (python:60,
php/c/ruby:35) declares this marker at module level and makes it importable;
the JS parser was the outlier, trapping the canonical boundary string in a
method body where no external consumer could reach it.

Move the declaration to module level next to the requires, reference it
unchanged inside _assembleEnhancedCode (resolves via lexical scope, output
byte-identical), and add it to module.exports. No runtime/behavior change.

Tests: new tests/parsers/javascript/test_unit_generator_file_boundary.py
(node-subprocess seam) asserts the export exists and equals the canonical
marker string. RED 2 failed pre-fix; GREEN 2 passed. Full suite 219 passed,
22 skipped, 0 failed; ruff + node --check clean.

Co-Authored-By: Claude Opus 4.7 (1M context) noreply@anthropic.com

[gadievron]

Closing as already resolved on current master. This fix's behavior shipped via the parser-fix-stack release (#133/#134); confirmed by independent reproduction — the original bug (reconstructed from its bug-pipeline investigation) no longer manifests on master, not merely by line-presence. Part of a full validated sweep of the open-PR corpus.