docs: add tap gesture combining guide

Author: AsafMah

docs/two-thumb-step-one.md

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+# Tap + Gesture Combining (Step One)
+
+LeanType can optionally accept a tap from one finger and a simultaneous gesture from another finger as part of the same current word. This is a small first step toward two-thumb typing support.
+
+The feature is intentionally conservative: it does not keep a word open after all fingers leave the keyboard, and it does not add spacing, grace timers, or delayed commit behavior.
+
+## Enable it
+
+Open **Settings → Gesture Typing** and enable:
+
+1. **Combine simultaneous tapping and additional gestures**
+2. Optional: **Draw taps and gestures**
+
+Both settings default to off.
+
+## How it works
+
+With the combine setting enabled:
+
+- A gesture may bypass the rapid-typing suppression only when another pointer is still active.
+- The previous input must have been a letter.
+- The normal upstream word boundary remains unchanged: once all fingers are lifted, the next gesture starts a new word.
+
+This means overlapping multi-pointer input is allowed, but a later single-finger tap → swipe sequence is not treated as part of the same word.
+
+## Example
+
+A supported sequence is:
+
+1. Press or tap a letter with one thumb.
+2. While at least one finger is still active, start a gesture with another finger.
+3. Lift normally to finish the gesture.
+
+An out-of-scope sequence is:
+
+1. Tap a word.
+2. Lift all fingers.
+3. Tap space.
+4. Start a new single-finger gesture.
+
+That remains normal gesture typing for the next word and keeps the usual rapid-typing guard.
+
+## Debug drawing
+
+The optional **Draw taps and gestures** setting draws a lightweight diagnostic overlay:
+
+- tap points are shown as points
+- gesture samples are drawn as a simple path
+
+The overlay is for manual testing and debugging only. It does not affect committed text or suggestions.
+
+The overlay clears when a new pointer sequence starts, when gesture input is cancelled, and when debug drawing is disabled.
+
+## Scope limits
+
+This step deliberately avoids the larger two-thumb model from downstream forks:
+
+- no phantom spaces
+- no grace timers
+- no delayed commit after all fingers lift
+- no re-recognition of prior taps
+- no custom spacing behavior
+- no synthetic point insertion
+
+Those can be evaluated separately after the minimal mixed-input gate has proven useful.
+
+## Implementation notes
+
+The feature adds two cached settings in `SettingsValues` and a small `TapGestureDrawingPreview`. `PointerTracker` passes a new `ignoreFastTypingCooldown` flag to `BatchInputArbiter` only when overlapping multi-pointer tap+gesture input is active.
+
+`BatchInputArbiter` keeps the original five-argument `addDownEventPoint` as the default path, so existing callers preserve upstream behavior.