feat: add terminal-theme color mode

[andy7415]

Summary

Adds a new --color terminal mode that uses the terminal's configured ANSI palette while preserving rmatrix's smooth fading behavior.

Changes

• Query the terminal's chromatic ANSI colors using OSC 4.
• Assign a random color to each rain column.
• Reuse the existing RGB fade and bright-head rendering.
• Add fallback colors for terminals without OSC 4 support.
• Support BEL and ST OSC response terminators.
• Validate RGB palette response formats.
• Add tests for parsing, fallback behavior, palette selection, and fading.
• Document the new option and terminal compatibility in the README.

Verification

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test

All checks pass.