Add axis mapping authoring model

[kostyafarber]

What changed

• model external/internal axes, continuous/discrete axis kinds, value labels, and font-owned mappings with stable IDs
• evaluate independent mappings before the single cross-axis mapping group in Rust
• persist mappings through the working store, .shift packages, undo/redo, N-API, generated TypeScript DTOs, and renderer workspace state
• round-trip Designspace per-axis maps, cross-axis mappings, discrete axes, and standard axis labels
• expose mapping mutation/query APIs to the renderer and full read-only mapping inspection in shift-cli

Why

The existing axis model only represented continuous min/default/max ranges. The axes UI needs one coherent authoring model for mapping lists, graph samples, discrete axes, axis value labels, and Fontra-compatible cross-axis mapping data.

Impact

• .shift axis records now use role, a discriminated kind, and labels
• .shift packages require axis-mappings.json
• schema version remains 1; this is a pre-release baseline change with no runtime migration path
• deleting or updating axes validates dependent mappings and preserves them correctly through undo/redo

Validation

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• pnpm typecheck
• pnpm lint:check
• pnpm test
• repository pre-commit suite, including formatting, dead-code checks, and Vitest