feat(core): derive missing ANSI 256 palette entries from base16

[kavhnr]

Summary

• Add CIELAB-based 256-color derivation from ANSI base16 in TerminalPalette, aligned with Ghostty's approach.
• Fill missing palette indices 16-255 when size >= 256 and base16 + fg/bg are available, while preserving any explicit palette entries already returned by the terminal.
• Add focused tests for derivation, non-overwrite behavior, and fallback defaults when OSC 10/11 are unavailable.

Context

This follows the Ghostty work and reference implementation notes:

• generate 256 palette ghostty-org/ghostty#10554
• https://gist.github.com/jake-stewart/0a8ea46159a7da2c808e5be2177e1783

Behavior

• New GetPaletteOptions.generateFromBase16 option (default: true).
• Generation activates only when:
  • size >= 256
  • ANSI 0-15 are present
  • foreground/background are available (from OSC 10/11, with fallback to palette 7/0).
• Existing detected values for 16-255 are never overwritten.

Validation

• bun test src/lib/palette-256.test.ts src/lib/terminal-palette.test.ts

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commit: 00deccc