Generate the theme catalog from real themes instead of a hand table

The derivation landed in decision 0018 with the catalog and the terminal ANSI
palette still fixed tables written by hand. tools/theme-gen reads the
TextMate themes under assets/themes/ and emits the catalog and the reference
vectors, so the anchors a palette is derived from are the ones the theme
actually ships rather than the ones somebody transcribed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SPYebLiN2w4TqnHUYGdECq

Cargo.toml

@@ -12,6 +12,7 @@ members = [
     "crates/lumbridge-terminal",
     "crates/lumbridge-theme",
     "crates/lumbridge-ui-fixture",
+    "tools/theme-gen",
 ]
 exclude = ["spikes"]
 resolver = "2"