subsys/input: add the input subsystem

[gmarull]

Adds subsys/input, a Zephyr-style input subsystem. Drivers report events with pbl_input_report() and know nothing about who consumes them. Listeners are defined with PBL_INPUT_CALLBACK_DEFINE() next to the code that uses them and are collected at link time, the same way shell commands are.

• Events: KEY (buttons, BTN_TOUCH), ABS (touch X/Y) and GES (tap, double tap, palm). Events are grouped into reports, and the last event of a report carries sync.
• Delivery is synchronous: listeners run in the context of the report. That is the debounce ISR for buttons, the system task for touch, and the calling thread for injected input. The subsystem has no queue or thread of its own, because the kernel event queue already moves work out of ISRs. Listeners must not block, and they check pbl_in_isr() before calling APIs that are not ISR-safe.
• Buttons: the four drivers (SF32LB, nRF5, QEMU, SDL) report key events. Each board's ButtonConfig sets the key code its button reports, so the codes are independent of ButtonId. A listener in fw/kernel maps the codes back to ButtonId and turns them into PEBBLE_BUTTON_DOWN/UP_EVENT, so click recognition and everything above it is unchanged. The SELECT+BACK hard reset stays in the drivers on purpose: it has to work when every thread is wedged.
• Touch: CST816, QEMU and SDL report samples and gestures. The touch service collects each report and passes it on to touch_handle_update() / touch_handle_gesture().
• Injection:
  • button click/hold/multi, the remote input endpoint and button raw all go through the subsystem. button raw was duplicated in three drivers; it now exists once and works on every board.
  • New input dump on|off and input report <key|abs|ges> <code> <value> [sync] shell commands.

Design notes are in docs/architecture/input.md.

Future work, not part of this PR: a generic gpio_keys driver that owns debouncing, the hold timer and combos, once the device model can describe keys per board.

Testing

• pbl test: 375/375 pass, including the new test_input and test_remote_input, which now runs through the subsystem and the button listener.
• Builds: qemu_emery, obelix@pvt (normal and PRF), asterix and native_emery on SDK 0.1.11. qemu_emery also builds at every commit.
• QEMU (qemu_emery), with input dump on:
  • QEMU sendkey and pbl touch produce the expected key, BTN_TOUCH and ABS reports.
  • In the launcher, button raw 3 1/0, button click 3 and input report key 3 1/0 each move the selection down one row.
• obelix (pvt), with input dump on:
  • BACK, UP, SELECT and DOWN report key codes 0–3 on press and release through the SF32LB debounce path.
  • The CST816 reports touch samples (BTN_TOUCH, ABS_X, ABS_Y) and the tap, double-tap and palm gestures.
  • button raw, button click and input report key report the expected events.
  • Buttons, touch scrolling and gestures drive the UI as before.
• Not tested on hardware: the nRF5 debounce path (asterix) is only build-tested.

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