input, render: keep fast touch typing whole, fire pattern nodes once, and make the software image path cheap

Two ways a tap was lost on a phone. The gesture machine serves one finger: a second finger landing while the first was still down went to the auxiliary slot and never reached it, so with two thumbs every overlapping tap on a keypad vanished. A finger landing on a primary that is still a plain press (GestureState::is_plain_press: no drag, scroll, swipe or slider in flight) now commits that press as a tap at its last position and takes the primary slot; the earlier finger's release reports through on_touch_up, and a second finger during a pattern stroke or a slider drag stays out as before. The touch-up body moves to release_primary so both paths share it. The other loss needed one finger: the 8 px travel that turns a press into a scroll, swipe or drag, and the 6 px stray that cancels a touch long-press, were compared against physical coordinates, so at buffer scale 2 they were 4 and 3 logical px and an ordinary thumb tap became a vertical drag whose release skipped the button. SwipeConfig carries the surface's scale and the machine reads the thresholds through slop() and stray(), which are logical.
Pattern grids fired on_node on every motion event within a node's snap radius, and the Slider move outcome requested a repaint each time, so drawing a pattern rebuilt and re-rastered the surface at the touchscreen's motion rate. The machine remembers the node under the finger (last_pattern_node) and emits only on change; the handlers split pattern_node_at from pattern_msg, with pattern_node_msg kept on top of them; a move outcome without a message no longer requests a redraw.
SoftwareCanvas::draw_image_data premultiplied the whole source in floating point on every call, so a full-surface backdrop paid a pass over every pixel per frame before the blit. Premultiplied copies now live in a 32 MiB least-recently-drawn cache keyed by size, content fingerprint and alpha, the key the GLES texture cache already used; fingerprint_rgba moves to render::helpers and both backends share it. The entry records whether the source was fully opaque, and an opaque image drawn 1:1 with no clip and full alpha is copied row by row into the canvas instead of going through tiny-skia's bilinear pipeline.
docs/architecture.md describes the primary-slot hand-over, docs/widgets.md the once-per-node contract, and the CHANGELOG lists the five fixes.

docs/architecture.md

@@ -320,7 +320,7 @@ Downstream consumers shipping into regulated environments (EN 301 549, WCAG 2.1
 
 **Cross-application drag-and-drop — deferred.** The clipboard now bridges to the Wayland selection via `wl_data_device_manager` (see `event_loop/data_device.rs`), so Ctrl+C / Ctrl+V crosses application boundaries when the compositor advertises the global. Middle-click primary selection (`zwp_primary_selection_v1`) and inter-app drag-and-drop targets (drop-zone widgets that accept text / URI lists from outside the process) are still pending — they share most of the offer / source plumbing but need widget-level drop-target wiring on top.
 
-**Multi-touch — primary slot plus raw auxiliary fingers.** The first finger to land on a surface becomes its primary slot and drives the built-in gesture machine (swipe, scroll, long-press, drag). Additional fingers bypass the gesture machine and surface directly through `App::on_touch_down` / `on_touch_move` / `on_touch_up`, so apps can implement pinch-zoom or two-finger pan without losing the built-in gestures. Apps that want the whole stream raw — an embedded web view, a drawing canvas — return `true` from `App::claims_raw_touch`: every finger, primary included, then reports through `on_touch_*` and widget presses, taps and swipes never fire. What ltk itself does not provide is *recognition* of multi-finger gestures — pinch and rotate detection is the app's job on top of the raw stream.
+**Multi-touch — primary slot plus raw auxiliary fingers.** The first finger to land on a surface becomes its primary slot and drives the built-in gesture machine (swipe, scroll, long-press, drag). Additional fingers bypass the gesture machine and surface directly through `App::on_touch_down` / `on_touch_move` / `on_touch_up`, so apps can implement pinch-zoom or two-finger pan without losing the built-in gestures. One exception keeps fast two-thumb typing whole: a finger landing while the primary is still a plain press (no drag, scroll, swipe or slider in flight) commits that press as a tap at its last position and takes the primary slot itself, so alternating taps whose touches overlap all register; the earlier finger's release then reports through `on_touch_up`. Apps that want the whole stream raw — an embedded web view, a drawing canvas — return `true` from `App::claims_raw_touch`: every finger, primary included, then reports through `on_touch_*` and widget presses, taps and swipes never fire. What ltk itself does not provide is *recognition* of multi-finger gestures — pinch and rotate detection is the app's job on top of the raw stream.
 
 **HarfBuzz shaping — wired in.** `src/text_shaping.rs::shape_line` now drives both renderers: the line is BiDi-reordered, split into per-font sub-runs and shaped through rustybuzz. The glyph cache is keyed on `(glyph_id, size_bits, font_id)` and each glyph is rasterised by index via `fontdue::Font::rasterize_indexed`, so Arabic connected forms, Devanagari clusters and CJK shaped glyphs render correctly.