vscode: intelligent live tint for palettes without a bundled theme

[xscriptor]

What

Makes VS Code and VS Code - Insiders follow any palette, including community/ and user-created ones, not only the 12 bundled in xscriptor-themes.

How it works (all dynamic, nothing palette-specific)

• New themesync/vscode_theme.py: semantic base16 → VS Code mapping. 269 workbench keys (surfaces mixed bg→fg, selection from the accent, terminal ANSI 0-15, git decorations, diff, badges, charts...), 24 TextMate token rules and 20 semantic token rules. Every foreground is WCAG-checked (AA), dark/light is derived from luminance, and optional palette roles (accent, mauve, teal, peach...) refine the accents.
• vscode target now decides per editor:
  • bundled palette (theme file found by globbing the installed xscriptor.xscriptor-themes-* extension): keep the handcrafted theme + icon theme, and remove the generated block;
  • any other palette: set a coherent base (Default Dark/Light Modern by luminance) and inject a marker-delimited block with workbench.colorCustomizations + editor.tokenColorCustomizations + editor.semanticTokenColorCustomizations. VS Code watches settings.json, so it re-tints live in every window.
• User-defined customization keys are never overwritten: if one exists outside the block, ours for that key is skipped.
• Adding themes/icons in the extension later (or renaming palettes) needs no code change: everything is discovered at runtime.

Testing

• scripts/check.sh: new step validates the mapping for all 137 palettes of the checkout (valid hex colors, WCAG AA contrast on every one; minimum measured 4.53), plus the existing compile/CLI smoke tests.
• Throwaway HOME end-to-end: dynamic path (valid JSONC, user keys preserved, idempotent), native path (block removed, theme+icons set, idempotent), and light-palette base selection.
• Real machine: applied for community palette 3024 (Insiders settings got the 269-key block, editor.background #090300), then switched back to x and the file was restored byte-identical to the backup.