docs: add the steering trio (STEERING.md, ADR.md, root CLAUDE.md)

[jonathanstokes]

Summary

Adds durable engineering context in three files with distinct jobs: docs/STEERING.md describes what the system is (boundaries, patterns, constraints) and points into architecture.md rather than duplicating it; docs/ADR.md records four long-standing decisions with the alternatives that lost and why; CLAUDE.md at the repo root carries the working rules an agent or contributor needs on every change. None of these ship to PyPI - they are not in the wheel's package dir or on the sdist include list, so they exist only in the git tree.

Changes

• docs/STEERING.md: purpose, scope and boundaries, owned paths, four patterns with canonical code examples, six measurable constraints, pointers, and the infrastructure picture
• docs/ADR.md: ADR-0001 committed protobuf bindings with the coupled runtime pin, ADR-0002 fully offline test suite behind a single lazy hid import, ADR-0003 USB HID as the only transport, ADR-0004 the domain model lands additively on the unchanged protocol layer
• CLAUDE.md: dev/test commands, the import-safety rule, the bindings/pin rule, the keyed-edit rule, evidence-bearing docstrings, the RX-thread invariant, and a Do-not list for the genuinely dangerous surfaces (Updater, cloud, unattended IR probing)

Test plan

• Full offline suite passes (323 passed, 3 skipped)
• Every relative link target in the three files resolves
• Every cited code symbol verified to exist (switch_scene, set_bypass, read_preset, set_param, FakeTransport, open_device, _open_unconnected)
• Publishing claim checked against releasing.md (token-based, not trusted publishing - corrected before this PR)