Add the core theme catalog

[konojunya]

Background

Stack 1.0 needs versioned visual inputs before the engine facade and deterministic layout work can begin. The theme package boundary existed, but its canonical catalog was still empty.

Implementation

• add the default, light, and dark core themes with complete Stack 1.0 node-kind fallbacks
• add 30 repository-authored Apache-2.0 SVG assets with stable logical subjects across themes
• make missing non-core theme and icon recovery explicit through default and kind-external
• add deterministic versioned font metrics, including ordered wide Unicode ranges for CJK and emoji
• extend schema, Rust, and TypeScript contracts and regenerate matching Cargo/npm artifacts
• add catalog completeness, contrast, non-color distinction, fallback, security, and artifact-parity tests
• add a dependency-free review page and record desktop/mobile visual and license evidence

Testing

• npm run validate
• npm run generate:check
• npm test (11 tests)
• npm run typecheck
• npm pack --dry-run --workspace @stack-sh/theme
• cargo +stable fmt --check
• cargo +stable test --workspace --locked
• cargo +stable clippy --workspace --all-targets --locked -- -D warnings
• RUSTDOCFLAGS="-D warnings" cargo +stable doc --workspace --no-deps --locked
• cargo +stable package -p stack-theme --locked
• cargo +1.85.0 test --workspace --locked
• cargo +1.85.0 clippy --workspace --all-targets --locked -- -D warnings
• Chromium visual review at 1440 x 1000 and 390 x 844 with no horizontal overflow, console errors, or page errors

Review focus / concerns

• verify that fallback shapes and icon subjects remain understandable without relying on color
• verify that fallbacks.missingThemeId and fallbacks.missingIconId are sufficient for the later diagnostic-preserving engine fallback flow
• the metrics intentionally provide deterministic layout advances rather than exact platform glyph geometry
• no registry publication is included