Define BargeKit turn-taking state machine

[rogerchappel]

Summary

• Defines the Wave 1 BargeKit turn-taking state machine for modes, states, inputs, transitions, events, and invariants
• Adds minimal state-machine constants and barge-in guard helper
• Adds tests that verify the contract surface and core invariants

Atomic commits

• docs: define BargeKit turn-taking state machine
• feat: publish BargeKit state machine constants
• test: cover BargeKit state machine contract

Validation

• npm test
• bash scripts/validate.sh

Task: bargekit-define-turn-taking-state-machine