v2.1: seed digitalio input change-detection from a last-known-state initial write in pinAdd

### Context

Deferred from the #943 review ([thread](https://github.com/adafruit/Adafruit_Wippersnapper_Arduino/pull/943#discussion_r3633871850), [deferral](https://github.com/adafruit/Adafruit_Wippersnapper_Arduino/pull/943#discussion_r3638605246)) as a nice-to-have for **v2.1**.

`DigitalIOHardware::CheckEvent()` (src/components/digitalIO/hardware.cpp) currently treats the first read after a pinAdd specially: `_prv_value` starts as a guess, so the first read seeds the baseline from hardware and is always published as the pin's initial state (fixed in dcb1f047 — previously the guess could suppress or fabricate a transition).

### Feature

For **input** pins, IO should supply the last known state in the pinAdd's optional initial `write` (the field already exists in `ws_digitalio_Add` and is currently only meaningful for outputs). The firmware would use that as the starting *previous value* for the on-change trigger:

- current reading == last known state → no redundant initial event; the broker already has this value
- current reading != last known state → a genuine **transition** event fires, so a state change that happened while the device was offline / rebooting / sleeping is detected and published as a change rather than a fresh baseline

### Notes

- Needs IO/broker cooperation: the pinAdd for an input must carry the last value the broker saw (companion change in adafruit/Wippersnapper_Protobuf semantics/docs if the existing `write` field is reused).
- Interaction with `is_inverted` and expander-backed pins (`EXP_0xNN_P`) should follow the logical-value convention used by `ReadValue()`.
- Sleep/wake replay makes this more valuable: every deep-sleep wake re-runs componentAdds (see #952), so today each wake re-publishes baselines for every input.

Refs: #943, #952.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start in src/components/digitalIO/hardware.cpp at DigitalIOHardware::CheckEvent(), then trace ws_digitalio_Add and ReadValue() to understand initial writes, logical values, inversion, and expander pins. Check the referenced Wippersnapper_Protobuf semantics/docs and componentAdds wake path; done means input pinAdd state suppresses redundant events and publishes an offline transition consistently with the broker.

Written by the indexing model from the issue text.