Spike: Galileo OSM basemap integration

[frewsxcv]

Summary

• Adds a new workspace crate rgis-basemap-spike that renders an OpenStreetMap basemap via Galileo into an offscreen wgpu texture and displays the RGBA bytes as a Bevy sprite tracking the rgis Camera2d.
• Proof-of-concept for landing a real basemap layer — this is not production code, it's a spike to validate that Galileo (wgpu 29) and Bevy 0.18 (wgpu 27) can coexist in one binary and that the CPU-bridged pipeline is workable.
• Native-only (#[cfg(not(target_arch = "wasm32"))]). wasm is deliberately out of scope for this spike.

Architecture

• A dedicated OS worker thread owns both a tokio multi-thread runtime and Galileo's WgpuRenderer for its entire lifetime. The main Bevy thread never touches either.
• The plugin communicates with the worker through std::sync::mpsc request/response channels. The worker drains its request backlog before each render (coalescing rapid camera moves to only honor the newest).
• Main-thread refresh_basemap system each Update:
  1. Drains pending RenderResponses with try_recv (latest wins) and replaces the sprite's Image asset in-place via images.insert(&handle, new_image). (An earlier version swapped to a fresh handle, which works in debug but races Bevy's asset extraction in release mode — keeping the handle identity stable fixes it.)
  2. Reads the Camera2d transform; if it has zoomed past SCALE_DELTA or panned past PAN_DELTA of the last rendered texture's half-width, queues a new render request (respecting in_flight + REFRESH_DEBOUNCE).
  3. Syncs the sprite transform to the latest render snapshot so the basemap sits at the correct world position and scale.
• The basemap sprite is parked at BASEMAP_Z = -0.05 — just inside Bevy 2D's default visible z range ([-0.1, 1999.9] with the camera at z=999.9) so rgis layers at z≥0 still draw on top.
• A tiny demo_jump_to_nyc system teleports the camera to NYC on frame 1 and Paris on frame 240 so you can see the basemap swap in without needing to click anything. Remove before this graduates to real code.

Release-mode fixes

Getting this working in release required two fixes landed in this branch:

• Stable release hits a trait-solver recursion overflow in geo-bevy (pre-existing in main; CI only builds desktop in debug so it's never been surfaced). Workaround: nightly with RUSTFLAGS="-Zinline-mir=no" (same workaround the WASM CI build already uses).
• Nightly release then fails because pathfinder_simd 0.5.5 (via galileo → font-kit) uses nightly intrinsics simd_fmin/simd_fmax that were renamed in simd_fmin/fmax: make semantics and name consistent with scalar intrinsics rust-lang/rust#154043. Crate is dead with no newer version. Worked around with a [patch.crates-io] entry pointing at PaulWagener/pathfinder which renames the intrinsics. This is a temporary hack — upstream (simd_fmin/fmax renamed breaking ARM builds in the future servo/pathfinder#584) needs a real fix.

Build command: RUSTFLAGS="-Zinline-mir=no" cargo +nightly build -p rgis --release

Known limitations / TBD

• Texture size is fixed at 1024². Should follow the window/viewport in a real integration.
• wasm support not attempted (no tokio multi-thread runtime on wasm; would need a different concurrency story).
• Real GPU texture sharing is blocked by the wgpu version skew between Bevy 0.18 and Galileo — this spike intentionally uses the CPU bridge (get_image() → RGBA bytes → Bevy Image) which works but is wasteful.
• RasterTileLayer is reconstructed on every render inside the worker. A real integration should keep it alive and just update the MapView.
• demo_jump_to_nyc should be deleted before merging.
• The pathfinder_simd patch is a temporary hack pending an upstream fix.

Test plan

• Debug build: cargo build -p rgis
• Release build: RUSTFLAGS="-Zinline-mir=no" cargo +nightly build -p rgis --release
• Run the binary, confirm NYC renders at frame 1, Paris at frame 240
• Drag the map — confirm basemap re-renders as the camera pans past thresholds
• Confirm no main-thread hitches during renders (worker renders are 30–120ms each in release, 400–1000ms in debug, all off-thread)
• /tmp/rgis-basemap-latest.png gets written on every render for visual verification