[RM2 2/3] Port demos to the shared runtime

[jimmystridh]

What changed

• add a compiled demo runtime for signal-aware waits, evdev discovery, display setup, CLI parsing, and pen configuration
• discover input devices by role, query pen ranges through EVIOCGABS, and apply the panel profile's coordinate transform
• migrate every demo to the canonical surface API, including format-safe RGB16/RGB32 pixel helpers and capability-gated color demos
• add a portable map renderer that saves and restores only the clipped backing patch
• build the archived vanishing-trail example for both supported targets

Why

The original demos duplicate Paper Pro-specific input names, coordinate limits, signal handling, and RGB32 framebuffer assumptions. Centralizing those boundaries makes the examples usable across RM2 and Paper Pro without weakening the core ABI.

Impact

RM2 now builds the interactive demos, including scribble, map_demo, and vanishing_trail. Color-only programs fail cleanly on monochrome panels. Input disconnection terminates the runtime instead of spinning.

Validation

• sanitizer-backed host tests on macOS and Linux
• Linux signalfd, evdev disconnect, timeout, and CLI boundary tests
• strict ARMv7 and AArch64 compilation of the shared runtime
• full RM2 SDK build of every demo

Series

This is 2 of 3 and depends on #2. GitHub requires the base branch to exist in this repository, so this draft targets main and temporarily includes the two core commits. The focused two-commit delta is jimmystridh/quill#2.