responsive: async loader, texture cache LRU, prefetch, tests (it0)

M3 (task it0): responsive + prefetch.

- src/loader/loader.{c,h}: loader_load_async/_finish wrap the sync
  loader_load in a GTask worker; the task's source object is the GFile so the
  finish callback can check it against navigator.current (last-write-wins).
- src/texturecache.{c,h}: bounded LRU (cap 4) of GFile->GdkTexture, hash+GQueue
  O(1) get/put, entry owns the key (no double-unref), evicts LRU on overflow.
- src/window.c: async visible load (cancel-then-recreate single GCancellable);
  cache hit -> show+prefetch, miss -> async load; last-write-wins guard
  (g_file_equal(current, loaded)) before showing; prefetch next/prev into the
  cache via a separate prefetch cancellable (cancelled each round). Finish
  callbacks hold a ref on the window (released in the callback) so they never
  deref a freed window; dispose cancels both cancellables.
- tests: unit test_texturecache (LRU evict/order/replace/miss), integration
  test_responsive_nav (10 rapid next -> last-write-wins). M1/M2 tests updated
  to pump the main loop (loads are now async) + drain_main before exit.
- meson: texturecache in libggae; GIO_USE_VFS=local in fixtures_env.

Fixed: texturecache double-unref (key ownership), async-callback UAF (window
ref in callback data + drain in tests), a test g_build_filename leak. Sub-agent
review issues addressed (dispose + cache-hit cancel in-flight loads).

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