Support SDL3 backend alongside SDL2

[jserv]

This builds the compat stack against either SDL major version from one source tree, selected by the SDL_BACKEND make variable. When unset, the build auto-detects: it prefers SDL3 when both sdl3 and sdl3-ttf are visible to pkg-config, and falls back to SDL2; an explicit SDL_BACKEND= sdl2|sdl3 always wins. SDL2 remains the wrapper-shim path (dlopen the host SDL2); SDL3 links libSDL3 / libSDL3_ttf directly.

Translation lives in two new chokepoint headers, src/sdl-compat.h and src/sdl-ttf-compat.h, that every source includes instead of <SDL2/SDL.h>. The SDL2 branch is a pass-through; the SDL3 branch enables SDL_ENABLE_OLD_NAMES (so SDL3's old-names layer carries the pure renames) and wraps only the symbols whose signature or semantics changed: the render float migration, surface mask->enum creation, the MapRGB/GetRGB palette argument, SDL_RenderReadPixels returning a surface, SDL_CreateWindow losing x/y, SDL_SetTextInputArea, mouse-coordinate floats, ns->ms event timestamps, the SDL_ttf 3.x renames, and the SDL2 int (0/-1) return convention the call sites test. Wrappers are defined before the #define that renames the call sites so their bodies bind the real SDL3 symbol.

mk/sdl.mk gains the SDL_BACKEND switch + auto-detect and a content-hashed SDL_BACKEND_STAMP that every object depends on, so switching backends forces a rebuild without manual object deletion. The SDL2 dlopen wrapper is gated off under SDL3 (mk/sdl-wrapper.mk), and the two wrappers' shared dlopen/dlsym/atomic-cache machinery is consolidated into src/wrapper/dlwrap.h.

Per-site SDL3 fixes: font.c pins the glyph texture to nearest scaling (SDL3 defaults to linear, which smeared opaque strokes); pixmap.c populates the render-target pixmap via a staging-texture render (SDL_UpdateTexture into a target did not survive readback on SDL3's software renderer); events.c restructures the flattened window events, key fields, and timestamps, and reconciles the per-display event-queue accounting at the client SDL_FlushEvent/PeepEvents/WaitEvent boundary.

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Summary by cubic

Add optional SDL3 backend alongside SDL2, selectable via SDL_BACKEND, with auto-detect that prefers sdl3 when available. Compat headers keep source changes small; CI adds an sdl3 job to exercise the new path.

• New Features

  • Build against SDL2 or SDL3 via SDL_BACKEND=sdl2|sdl3; auto-detect prefers sdl3/sdl3-ttf, unknown values fail fast.
  • Compat headers: src/sdl-compat.h and src/sdl-ttf-compat.h unify APIs (SDL_ENABLE_OLD_NAMES + small wrappers for changed calls/returns); all sources and examples include them.
  • Build: SDL_BACKEND_STAMP forces rebuilds on backend switch; sdl2 uses dlopen shims, sdl3 links libSDL3/libSDL3_ttf; shared dlopen/dlsym logic moved to src/wrapper/dlwrap.h.
  • CI: new sdl3 job builds and tests the SDL3 path from source, caches the prefix, and runs headless; other jobs stay on sdl2.
  • SDL3 fixes: nearest scaling for glyph textures; pixmap targets populated via a staging texture; updated event fields and ms timestamps.
  • Security: replaced banned strcpy in the text-event shim with bounded snprintf.

• Migration

  • To use SDL3: install sdl3 and sdl3-ttf, then build with SDL_BACKEND=sdl3.
  • To force SDL2: build with SDL_BACKEND=sdl2; auto-detect picks sdl3 when both sdl3 and sdl3-ttf are available.
  • Runtime: sdl2 uses wrapper shims; sdl3 links libSDL3/libSDL3_ttf directly.

^{Written for commit 9046ba1. Summary will update on new commits.}