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

tools/theme-gen/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "theme-gen"
+description = "Dev-only generator that turns the vendored TextMate themes into lumbridge-theme's catalog"
+publish = false
+version.workspace = true
+edition.workspace = true
+rust-version.workspace = true
+license.workspace = true
+repository.workspace = true
+
+[dependencies]
+lumbridge-theme = { path = "../../crates/lumbridge-theme" }
+serde_json = "1.0"
+
+[lints]
+workspace = true