Implement palette library (.gpl) and per-drawing schemes (ADR-0002) #4

Summary

Implement palette and scheme support for drawings: load GIMP Palette (.gpl) files from a user-managed library, and let each drawing turn a palette into a named color scheme.

The design is fully decided and recorded in ADR-0002 (see PR #3, doc/adr/ADR-0002-Palettes-and-Schemes.md). Read it first; this issue is the implementation handoff and does not repeat the full rationale.

Background: Palette vs Scheme

• Palette: an ordered set of colors (optionally named), stored as a .gpl file in the user's library. Re-usable, carries no semantics.
• Scheme: a palette plus a mapping of user-chosen role names (bg, main, accent, ...) to the palette's colors. Created in drawing code, used there, never stored.

The flow in a drawing: load palette → map roles in drawing code → use scheme's named colors.

Scope of this issue

1. .gpl parser — our own, minimal, against the official spec (formalized 2023, MUST-level rules): https://developer.gimp.org/core/standards/gpl/
   • First line MUST be GIMP Palette
   • Optional Name: line (UTF-8, stripped), optional Columns: line (0-255, cosmetic only)
   • Old, header-less format: if line 2 is not Name:, colors start at line 2 and the palette name is the file basename (spec-prescribed fallback)
   • Color lines: r g b integers 0-255, then optional color name (rest of line, stripped)
   • # comment lines and empty lines are ignored
   • Known replacement if ours proves insufficient: anypalette (multi-format, no TS types, browser-oriented) — do not add it now, the parser is an implementation detail behind the public API
2. Palette library — user-managed; opinionated default ./palettes/ in the drawing project (same convention as saves/). Vormen reads, never writes. No palettes shipped with the library. Users can copy any existing .gpl (GIMP, Inkscape, Lospec exports, public collections) there.
3. Palette identity — palette name from the embedded Name: header; filename basename fallback for old-format files. Names must be unique in the library; the loader fails loudly on duplicates or missing palettes.
4. Scheme creation in drawing code — an API to load a palette by name and map roles to its colors (by color name and/or index). Roles are free-form, user-chosen names. Exact API shape (function/class, module location, scheme.get("bg") vs scheme.bg) is deliberately postponed in the ADR — decide during implementation, keep the public API small per AGENTS.md.
5. Settings integration — a drawing declares a scheme setting whose value is a palette name; CLI override: --setting.scheme=wet-tokyo (singular setting., matching the existing runner convention in src/vormen/vormen.ts). Note: the setting selects a palette; the scheme exists only after the drawing assigns roles.
6. Colors as strings — expose colors as #RRGGBB strings, passed directly to svg.js fill/stroke/gradients. Full color utilities (HSL/RGB/OKLCH, lighten(10), etc.) are explicitly out of scope (postponed in the ADR).

Test fixtures required

At least two .gpl files for unit and/or integration tests:

• one with a proper Name: header and named colors (new format)
• one old-format file (no Name: line) to exercise the filename-basename fallback

Constraints (from AGENTS.md / ADR-0002)

• Tests first; each module has unit tests, each feature an integration test
• deno run check:all must pass with no errors or warnings
• Keep the public API minimal; do not export what has no use outside the module
• No new dependencies (no anypalette, no color libraries)
• Do not add manual lint/typing overrides
• Document the feature in README (new module section) and add Palette/Scheme to doc/glossary.md (the glossary already mentions "Color: Color manipulation and palettes" — extend it with the canonical terms)
• Postponed by the ADR (do not build): storing schemes, sidecar files, color manipulation utilities, serve-mode swatch widget, create-vormen scaffolding of palettes/, user-level palette dir, listing available palettes

References

• ADR: PR #3 → doc/adr/ADR-0002-Palettes-and-Schemes.md
• .gpl spec: https://developer.gimp.org/core/standards/gpl/
• Existing settings conventions: src/settings/settings.ts, src/vormen/vormen.ts
• Example palette collections in .gpl (for realistic fixtures): https://github.com/denilsonsa/gimp-palettes