futari v1.5.0 — family & friends test round

• Bundles gain gamecontrollerdb.txt — the community
  SDL_GameControllerDB
  (Zlib licence, snapshot 629ccc7, 2,254 mappings, tracked at
  dist/gamecontrollerdb.txt). The SDL input driver loads it from beside the
  exe, adding recognition for gamepads SDL3 doesn't know natively, and the
  startup warning about the file being absent goes away. Listed in
  THIRD-PARTY-NOTICES.txt.

• The SDK is vendored. thirdparty/rexglue-sdk is no longer a submodule
  pointing at a private fork — the tree itself is in the repo, taken verbatim
  from the fork at v0.9.3.1 (see thirdparty/rexglue-sdk/VENDORED.md for the
  documented deviations). A public clone now resolves every dependency.

• One ini again. v1.4.2 split the overlay's save target into
  futari_cvars.ini because the SDK's save was a truncating flat rewrite. The
  save now goes through a merge writer: F4's "Save to config" updates the
  friendly lines in futari.ini in place (comments and [Storage] intact) and
  keeps any other overlay-modified cvars in a marked block at the top of the
  file. A leftover futari_cvars.ini is loaded once at startup and parked as
  .bak. The write is temp-file + rename, so a crash mid-save cannot truncate
  the file.

Known issue building this exact tag on macOS: the vendored SDK's VERSION file collides with C++20's #include <version> on case-insensitive filesystems. Fixed on main immediately after the tag (file renamed REXGLUE_VERSION); the macOS bundle is built from that commit. Windows and Linux are unaffected.