Avoid allocating color name arrays for lookup

[OskarEichler]

Summary

Use the immutable color-name hashes directly for membership checks instead of allocating NAMES.keys arrays and linearly scanning them for every color lookup.

Public color_names results, ANSI-over-X11 precedence, errors and generated SGR output are unchanged.

Verification

• Existing suite: 220 examples, 0 failures on Ruby 3.2.11 and 4.0.6.
• RuboCop: 15 files, 0 offenses on both Rubies.
• External model: 763 behavior checks per Ruby across every ANSI/X11 name, both grounds, constructor/factory/disabled paths, precedence, invalid names and public-list isolation.
• Focused Ruby 4 microbenchmark, 20,000 calls/sample: X11 median 41.51 -> 21.63 ms and 400,000 -> 340,000 allocations; disabled X11 11.34 -> 1.66 ms and 40,000 -> 20,000 allocations; ANSI 13.19 -> 12.31 ms and 260,000 -> 220,000 allocations. Ruby 3.2 showed comparable allocation reductions. This is a focused microbenchmark, not application throughput.
• Rebuilt gem: 14 expected files and successful packaged loads on both Rubies.
• No test files changed.

Compatibility

No breaking change.