core: add tri-state key (start / continue / interrupt)

[rgoulter]

Summary

New key::tri_state family for a session that owns a modifier across later presses of the same key (Alt-Tab / swapper / ZMK tri-state).

• Start (first press): virtual-hold hold, tap tap. First HID report is the chord (e.g. Alt+Tab), not a naked Tab.
• Continue (re-press of the same key): tap tap again; hold stays down.
• Interrupt (any other resolved key): release hold in the same turn, so the host sees A, not Alt+A.

The physical key is NoOp; HID comes from VirtualKeyPress / VirtualKeyRelease. At most one session is armed.

Authoring

K.tri_state.alt_tab
K.tri_state.cmd_tab
K.tri_state.ctrl_tab
K.tri_state.custom { hold = K.LeftCtrl, tap = K.Tab }

Not in this PR: timeout, layer-off end, reverse/continue allow-list (Shift+Tab), swallowing the interrupting key.

Test plan

• cargo test -p smart-keymap-core --lib key::tri_state
• cargo test --test rust-integration tri_state
• cargo test -p smart-keymap-full-system-std --test cucumber-keymap -- -n 'Alt'
• just check-quick
• CI green

[Copilot]

Pull request overview

Adds a new key::tri_state key family to smart-keymap-core (and the Nickel authoring/codegen pipeline) to support “swapper” behaviors like Alt-Tab / Cmd-Tab: first press starts a session (virtual-hold modifier + tap key), re-press continues (tap again), and any other resolved key interrupts (release the held modifier).

Changes:

• Implement smart-keymap-core/src/key/tri_state.rs with a single armed session tracked in a per-family Context, emitting VirtualKeyPress/VirtualKeyRelease events.
• Extend Nickel authoring + codegen to support K.tri_state.* constructors and emit the corresponding Rust Key/System data.
• Add Rust integration tests and cucumber feature coverage wiring for the new family.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/rust/tri_state.rs	New Rust integration tests covering start/continue/interrupt behavior and custom hold/tap.
tests/rust/keymap.rs	Registers the new tri_state test module.
tests/rust/hid_keycodes.rs	Adds HID constants needed by tri-state tests (Tab, LAlt, LGUI).
tests/cucumber/keymap.rs	Adds tri-state key data to cucumber system construction (KeyVecs + System::new).
smart-keymap-full-system-std/tests/keymap_full_system.rs	Adds tri-state system slot to full-system test keymap assemblies.
smart-keymap-core/src/key/tri_state.rs	Core tri-state implementation: session management + virtual press/release injection.
smart-keymap-core/src/key.rs	Exposes the new tri_state module in the core key family list.
ncl/smart_keys/tri_state/keymap-ncl-to-json.ncl	Nickel → JSON mapping/validation for tri-state key specs ({ hold, tap }).
ncl/smart_keys/tri_state/keymap-codegen.ncl	Codegen rules for tri-state key data + system construction.
ncl/smart_keys/tri_state/key-extensions.ncl	Adds K.tri_state.{custom,alt_tab,cmd_tab,ctrl_tab} authoring helpers.
ncl/keys.ncl	Enables the tri-state key extension in the global keys.ncl fold.
ncl/keymap-ncl-to-json.ncl	Wires tri-state into the overall keymap NCL → JSON pipeline.
ncl/keymap-codegen.ncl	Wires tri-state into the overall Rust codegen pipeline.
ncl/key-extensions.ncl	Imports tri-state key extensions into the global extension set.
ncl/key_system/keymap-codegen.ncl	Adds TriState to generated key_system module lists/checks.
ncl/key_system/families.ncl	Defines the tri-state family in the generated aggregate system (data lengths, types, context expr).
features/keymap/key/tri_state.feature	Adds cucumber feature/spec text for tri-state behavior.

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