[RM2 1/3] Add model-aware core surface support

[jimmystridh]

What changed

• add a canonical quill_surface API with model, capability, format, geometry, and pen-orientation metadata
• support the reMarkable 2 ARMv7/Qt ABI, including the 32-bit external-buffer QImage constructor aliases
• validate vendor framebuffer layout before exposing it and support RGB32 and RGB16/RGB565 monochrome surfaces
• add an RM2 SDK build target and portable core/device probes while retaining Paper Pro compatibility
• strengthen clean-room, ABI, and release checks

Why

Quill's original proof of concept assumes the Paper Pro's AArch64 ABI, RGB32 surface, and private Qt constructor signature. The reMarkable 2 exposes the same high-level vendor framebuffer interface, but uses ARMv7 hard-float and a 32-bit stride ABI. This establishes the model-aware core boundary before porting interactive demos.

Impact

QUILL_SDK=/path/to/rm2-sdk ./build.sh rm2 builds libquill.so and the device probes for RM2. Interactive RM2 demos are intentionally left to the next PR in the stack.

Validation

• macOS sanitizer-backed host tests
• release and redistribution checks
• clean RM2 SDK cross-build producing ARM EABI5 hard-float artifacts
• verified 32-bit QImage C1/C2 aliases and additive surface/capability exports

Series

This is 1 of 3. The following PRs add the portable demos and crash-safe panel lifecycle. The focused fork-local review is jimmystridh/quill#1.