Clarify PCA9685 digital output and solenoid semantics

[BanjoR]

Automatic issue closing

Fixes #354

Scope

Addresses issue #354 by adding digital level writes to IO_PCA9685pwm: HIGH maps to 100% duty (4095) and remains held until LOW (0) is written. Both PCA9685 drivers explicitly clear every configured channel during initialization. The documentation defines the level-held semantics, bounded activation responsibility, safe external-driver boundary, and maintainer bench criteria.

Files changed are limited to:

• IO_PCA9685.cpp
• IO_PCA9685pwm.h
• docs/PCA9685_OUTPUTS.md
• tests/test_pca9685_output_contract.py

Personal integration code and unrelated protocol behavior are explicitly out of scope.

Existing-PR audit: PR #509 is an unrelated I2C-address correction and is conflicting; no superseded PCA9685 digital-output PR was identified.

Verification

• python -m pytest -p no:cacheprovider tests -q: 4 passed.
• Focused python -m pytest -p no:cacheprovider tests/test_pca9685_output_contract.py -q: 4 passed.
• Bundled Python syntax compilation of the regression test: passed.
• git diff --check: passed.
• Upstream ancestry (upstream/master is an ancestor) and exact changed-file allowlist: passed.
• Git tracked status: clean. An ignored generated .pytest_cache/ directory remains physically present because ownership permissions prevented removal; no generated file is part of the PR diff. This physical-artifact hygiene point is not claimed as fully cleared.
• Exact-head fork CI: PASS - CI run 31950094674 completed successfully for head 337aa5d.
• The upstream PR has no firmware check attached because the repository workflow is push-triggered; no required-check set was available through the branch-protection endpoint (HTTP 404).

Validation update:

• PASS - Exact default python -m platformio run in a clean task checkout with an isolated PlatformIO core completed for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI.
• No standalone native C++/format/docs toolchain was available for additional gates; the focused Python contract tests remain the host-side evidence.

Hardware validation: Not run—no hardware available.

Maintainer bench criteria

See docs/PCA9685_OUTPUTS.md. Reproducibly verify startup and independent-reset outputs remain off, HIGH is held until LOW, SDA/SCL and command-station power loss de-energize the external driver, repeated bounded activations stay within measured on-time/current/temperature ratings, and I2C recovery creates no unintended pulse.