Fix iOS keyboard launch crash and canvas visibility

[willwade]

Problem

The iOS keyboard extension was unusable. Three independent failure modes stacked on top of each other:

1. Hard jetsam kill on launch — exceeded mem limit: ActiveHard 77 MB (fatal) ~1s after the system swapped to the Dasher keyboard. The host app silently fell back to the default keyboard.
2. Canvas collapsed to zero height within ~10ms of appearing — even when we got past the memory cap, iOS shrunk the canvas to nothing because the extension didn't declare a desired height, leaving only the 44pt toolbar visible.
3. Empty PPM — with no alphabet visible, even a working canvas looked blank.

Each had to be fixed before the keyboard would stay on screen for more than a few hundred milliseconds.

Root causes (verified via Console.app on a physical device)

• The 311KB training_english_GB.txt produced a PPM trie that, combined with UIKit overhead and the alphabet/colour XMLs, blew the 77MB keyboard extension memory cap.
• IsFileWriteable() in DasherCore opened every bundled file in append-write mode just to test writability, generating a Sandbox: deny file-write-data for each colour/alphabet XML during Realize(). Fixed in DasherCore #20, now merged.
• The UIInputViewController's view had no explicit height constraint, so iOS measured the extension's required height as just the toolbar (44pt) and collapsed the canvas.
• The CADisplayLink retained its target via target: self, leaking the canvas across open/close cycles and firing NewFrame() 60–120 times/sec, expanding the model and growing memory until iOS shrunk us.
• The default alphabet was always English regardless of the user's system locale.

What's in this PR

Memory

• Trim keyboard training text from 311KB → first 50 lines (~24KB) in the Prepare Keyboard Data build phase (project.yml).
• configureForLowMemory() pins LP_NODE_BUDGET=200, LP_LM_MAX_ORDER=4, BP_LM_ADAPTIVE=0 so DasherApp's larger values don't leak into the extension.
• didReceiveMemoryWarning() no longer calls bridge.reset() (that rebuilds the node tree — opposite of what a memory warning needs).

Canvas visibility

• Add an explicit Auto Layout height constraint (320pt, .defaultHigh priority) on the input view controller's view. This was the last piece that turned a flash-then-disappear into a stable, usable keyboard.
• CADisplayLink uses a weak DisplayLinkProxy (no more retain cycle), is invalidated in deinit.
• preferredFramesPerSecond = 30, and tick() is gated on a needsRedraw flag set by touch handlers — stops the per-frame NewFrame() expansion that grew memory until iOS shrunk us.
• viewWillAppear calls bridge.reset() so each open starts at the root.

Locale + UX

• On first run (no dasher_settings.xml in the App Group), pick the alphabet from Locale.current rather than always defaulting to English. Currently de → German alphabet; everything else stays English. Extensible as more alphabets are bundled.

Diagnostics (kept — cheap, useful for future device bring-up)

• os_log breadcrumbs through DasherBridge.init, Realize(), setScreenSize, the first layout pass, first CADisplayLink tick, first draw, and canvas-height changes. Console.app filter: subsystem at.dasher.Dasher.keyboard.

Submodule bump

DasherCore: 5533eb9 → 3ddedde2 — pulls in DasherCore #20 (IsFileWriteable rewrite), which removes the sandbox-deny storm the keyboard generated against every bundled alphabet/colour XML during Realize(). That change is platform-neutral and strictly safer for every DasherCore frontend (Windows, GTK, WASM included).

Settings sync (free win)

Most user-facing settings already sync automatically: both DasherApp and the keyboard read/write the same dasher_settings.xml in the shared App Group (group.at.dasher.Dasher). Alphabet, palette, speed, input filter, locale, font — all carry over. The only intentional overrides are the LM memory-pressure params above.

Out of scope (deliberately)

• Control mode in the keyboard: tried, but most of its commands (select-all, paste, edit menu, speak) don't actually work in iOS keyboard context, and the control tree adds ~10MB of memory that re-triggers the jetsam kill. Worth revisiting as a separate feature if/when there's a proper keyboard-friendly control.xml + action callbacks wired to textDocumentProxy.
• Live locale-change handling: the alphabet is only re-picked on first run. If the user changes their iOS system locale later, the saved alphabet stays. Easy follow-up if needed.

Verification

Built and tested on iPhone (iPhone 12-class, iOS 18). Keyboard now:

• Stays alive across multiple open/close cycles
• Renders the full alphabet with colours from the user's chosen palette
• Picks the German alphabet automatically on a German-locale device on first run
• Honours alphabet/palette/speed changes made in DasherApp on the next keyboard launch