docs & HSL: update HackC frontend refs to Rust and expand unit test coverage

[omen18]

Description

This PR contributes documentation updates, configuration spec fixes, and 550+ lines of new unit test coverage across the Hack Standard Library (HSL).

1. Developer Documentation & Config

• Updated hphp/doc/hackers-guide/README.md and hphp/doc/hackers-guide/glossary.md to state that the Hack Compiler frontend (hackc) and bytecode emitter are written in Rust (updating obsolete references to OCaml).
• Corrected grammar and phrasing in hphp/doc/ini.md for wildcard configuration setting symlinking and copying.

2. Hack Standard Library (HSL) Test Coverage

• Math: Created MathExtendedSuiteTest.php and updated MathCompareTest.php with tests for trigonometric identities, exponential/logarithmic inverse pairs, base conversions, rounding modes, integer division exceptions, and float comparison edge cases.
• Str: Created StrExtendedSuiteTest.php and updated StrIntrospectTest.php with tests for chunking, trimming, padding, case folding (uppercase, lowercase, capitalize), multibyte emoji prefixes, splitting/joining, and string replacement.
• Vec: Created VecExtendedSuiteTest.php and updated VecSelectTest.php with tests for concat, fill, filter, map, reverse, sort, zip, and range.
• Keyset & Dict: Added test tuples to KeysetIntrospectTest.php and DictIntrospectTest.php for empty keysets/dicts, string key mappings, and null value handling.
• Regex: Added pattern string representation tests to RegexTest.php.

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[omen18]

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