feat(editor): Rainbow brackets and semantic highlighting (rainbow variables) (#59)

Two editor toggles under Settings › Editor › Color Scheme › Language Defaults (off by default, also actions `editor.toggleRainbowBrackets` / `editor.toggleSemanticHighlighting`):
- Semantic highlighting (IDEA's rainbow variables): locals and parameters coloured by a name hash from LSP semantic tokens; fields, statics, constants, library symbols untouched.
- Rainbow brackets (VS Code's bracket pair colorization): (), [], {} by depth, unmatched in the error colour, strings/comments skipped, visible ranges only with 4 KB checkpoints.
Palettes are theme tokens (`--syn-rainbow-1..5`, `--syn-bracket-1..3`, light/dark/high-contrast), editable in the Color Scheme tree, and mapped from IntelliJ `.icls` RAINBOW_COLOR1..5.

App verification on macOS found the colours were overridden by the syntax highlighter's span nested inside the marks; the rules now cover descendants. Verified both toggles on/off live in a TS file.

Fixes #56

https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

scripts/theme/intellij.ts

@@ -135,6 +135,11 @@ export const ICLS_ATTRIBUTES: Record<string, Array<[Part, string]>> = {
   DEFAULT_ATTRIBUTE: [["FOREGROUND", "--syn-attribute"]],
   TODO_DEFAULT_ATTRIBUTES: [["FOREGROUND", "--syn-todo"]],
   BAD_CHARACTER: [["FOREGROUND", "--syn-invalid"]],
+  RAINBOW_COLOR1: [["FOREGROUND", "--syn-rainbow-1"]],
+  RAINBOW_COLOR2: [["FOREGROUND", "--syn-rainbow-2"]],
+  RAINBOW_COLOR3: [["FOREGROUND", "--syn-rainbow-3"]],
+  RAINBOW_COLOR4: [["FOREGROUND", "--syn-rainbow-4"]],
+  RAINBOW_COLOR5: [["FOREGROUND", "--syn-rainbow-5"]],
   ERRORS_ATTRIBUTES: [["EFFECT_COLOR", "--wave-error"]],
   WARNING_ATTRIBUTES: [["EFFECT_COLOR", "--wave-warning"]],
   NOT_USED_ELEMENT_ATTRIBUTES: [["FOREGROUND", "--fg-unused"]],