feat(spec): add InteractionSurface contract for keyboard-first surfaces

[mdheller]

Summary

• add a canonical keyboard-first interaction contract family to sourceos-spec
• introduce InteractionSurface, CommandBus, FocusState, FocusTransition, and KeymapProfile as first-class typed contracts
• add concrete example payloads and short contract-addition notes for each tranche

Files added

• schemas/InteractionSurface.json
• schemas/CommandBus.json
• schemas/FocusState.json
• schemas/FocusTransition.json
• schemas/KeymapProfile.json
• examples/interaction_surface.json
• examples/command_bus.json
• examples/focus_state.json
• examples/focus_transition.json
• examples/keymap_profile.json
• docs/contract-additions/interaction-surface.md
• docs/contract-additions/command-bus.md
• docs/contract-additions/focus-state.md
• docs/contract-additions/focus-transition.md
• docs/contract-additions/keymap-profile.md

Intent

This PR places the first canonical cross-surface keyboard-navigation contract family in the correct repo: SourceOS-Linux/sourceos-spec.

The current family covers:

• typed interaction surfaces
• typed command routing / command-bus semantics
• typed focus ownership state
• typed focus transitions
• typed keymap/modifier profile binding
• host-boundary preservation vs mirroring vs opt-in replacement

Non-goals

• does not yet implement Linux-side runtime behavior directly
• does not yet define every possible browser/editor/app-local shortcut surface
• does not yet define downstream execution-plane evidence contracts for these interaction types

Follow-up

• bind a Linux-side implementation repo against these contracts without scattering runtime ownership across unrelated workstreams
• keep browser / office / Mac-on-Linux streams as downstream consumers of the canon rather than separate canonical owners
• extend the family further only when the first Linux-side binding exposes real gaps