Overhaul Azoteq IQS5XX touchpad scroll and drag behavior

[rnewman]

It is, in general, impossible for a QMK keyboard to implement smooth touchpad
scrolling on macOS: it ignores supplied resolution multipliers, doesn't ask for
reports, and seemingly only allows Magic Touchpads to deliver a good
experience. The recommended configuration is to use an app like SmoothScroll or
Mos to convert mouse wheel-style scrolls into smooth scroll; the work expected
of the firmware itself is to supply a reasonable range of scroll values.

This commit does so, though you might still wish to adjust configured values.

It includes three major changes to improve touchpad usability on macOS:

1. Configurable scroll speed reduction via axis_scale: replace naive integer
   division (which truncated small per-report values to zero, causing a
   "dead zone then jump" effect) with a remainder-preserving accumulator.
   Uses CONSTRAIN_HID_HV (new macro) instead of CONSTRAIN_HID: clamps to the
   proper HV range (int16 when WHEEL_EXTENDED_REPORT is defined, int8 otherwise).

2. Natural scrolling by default: invert the Y axis for two-finger scroll gestures
   so that content follows finger direction (swipe down = scroll down),
   matching macOS trackpad convention.

3. Replace press_and_hold drag with tap-to-drag:

   Problem: press_and_hold fired BUTTON1 whenever a finger lingered on
   the pad beyond HOLD_TIME, making it impossible to move the cursor
   without accidentally drag-selecting. Disabling press_and_hold at the
   IQS5XX hardware level caused the gesture engine to stall and stop
   detecting taps entirely.

   Solution (two parts):

   a) Decouple hardware gesture detection from click behavior. A new
   AZOTEQ_IQS5XX_PRESS_AND_HOLD_AS_CLICK define (defaulting to
   PRESS_AND_HOLD_ENABLE for backward compat) controls whether
   press_and_hold events produce BUTTON1 in the report handler.
   The hardware gesture stays enabled so the IQS5XX state machine
   remains healthy, but we simply ignore the event.

   b) Implement macOS-style tap-to-drag: after a single_tap, BUTTON1
   is held continuously for a configurable window (TAP_DRAG_WINDOW_MS,
   default 200ms). If a finger touches the pad again within that
   window, drag mode engages and BUTTON1 remains held until the
   finger lifts. The continuous hold during the window is critical —
   an earlier implementation that released BUTTON1 between tap and
   drag worked for text selection but failed for macOS window
   dragging, which requires an unbroken button press from the
   initial click on the title bar.

   TAP_TIME raised from 100ms to 200ms since there is no longer a need
   to keep it short to disambiguate taps from press_and_hold.
   HOLD_TIME remains at default 300ms.

New configurable defines (all with backward-compatible defaults):

• AZOTEQ_IQS5XX_SCROLL_DIVISOR (default 1, configured to 8 for Svalboard)
• AZOTEQ_IQS5XX_PRESS_AND_HOLD_AS_CLICK (default matches ENABLE)
• AZOTEQ_IQS5XX_TAP_DRAG_ENABLE (default false)
• AZOTEQ_IQS5XX_TAP_DRAG_WINDOW_MS (default 200)