docs: define general GBA usability milestones

docs/status.md

@@ -169,6 +169,12 @@ Writes to cartridge addresses return errors instead of modeling cartridge hardwa
 
 ## Next steps
 
+Follow the [general GBA usability roadmap](usability-roadmap.md).
+Prioritize repeatable homebrew gameplay, demonstrated compatibility failures, cartridge saves, and audio.
+The goal is usable emulation across games, not an Emerald-specific implementation.
+
+The remaining hardware-validation backlog includes:
+
 1. Extend shared fetch samples to verified general-bus data-access history and unsupported fetch regions.
    General DMA open-bus reads and sub-instruction arbitration remain separate from retained channel data and local IWRAM lanes.
    Keep unknown history diagnostic until fetch ordering and latch ownership have independent coverage.
@@ -183,7 +189,7 @@ Writes to cartridge addresses return errors instead of modeling cartridge hardwa
 3. Validate keypad retrigger behavior with hardware tests and add remaining device registers; extend BIOS reset coverage as sound/serial support becomes available.
    Validate STOP entry/wake edges and add external wake sources and remaining DMA device modes.
 4. Replace nominal sprite work limits with verified individual fetch timing; add background fetch timing and per-pixel composition.
-5. Validate more original/public test ROMs through the window; add audio, cartridge hardware, and saves before testing Emerald compatibility.
+5. Validate more original/public test ROMs through the window. Expand compatibility evidence across independent programs.
 
 Keep the emulator core independent of window and audio libraries.
 Treat Nintendo DS support as a separate project phase.

docs/usability-roadmap.md

@@ -0,0 +1,46 @@
+# General GBA usability roadmap
+
+The goal is a general-purpose GBA emulator, not a port of one game.
+Pokémon Emerald is one possible future compatibility target. It does not define hardware behavior.
+Nintendo DS support remains a separate phase.
+
+## Usability criteria
+
+A usable release must demonstrate these results with several lawfully obtained games:
+
+- Boot into a menu and enter gameplay without an unsupported-hardware diagnostic.
+- Present stable graphics in the native Darwin window.
+- Respond to button presses, held buttons, and releases.
+- Preserve supported cartridge saves across process restarts without damaging existing files.
+- Produce audio through an isolated host output layer.
+- Exit cleanly and report unsupported behavior clearly.
+
+A bounded run without a diagnostic is not proof of playability.
+A test pass establishes only the configured assertions, not complete game compatibility.
+
+## Work order
+
+1. Establish repeatable gameplay checks using licensed public homebrew.
+   Add VBlank completion, button schedules, and captured-pixel assertions to the ROM suite runner.
+2. Exercise menus and gameplay in the native window. Record the exact ROM revision and checksums.
+3. Select another independent program. Fix demonstrated boot, graphics, or input failures with original regressions.
+4. Implement cartridge save devices and safe host persistence. Verify saving and reloading across processes.
+5. Implement audio devices and host output. Keep host dependencies outside `gba-core`.
+6. Expand the compatibility matrix across games and hardware features.
+   Refine timing when independent tests or gameplay failures show a requirement.
+
+SRAM, Flash, and EEPROM require different protocols. Supporting one does not establish support for the others.
+Audio needs device clocks, mixing, and buffering; accepting sound-register writes alone is not audio support.
+The detailed hardware limits remain in [status.md](status.md).
+
+## Development loop
+
+For each feature, inspect the source and hardware evidence, then write original regression tests.
+Implement the smallest general behavior that satisfies the requirement.
+Run focused tests, workspace validation, and relevant native-window checks through `direnv exec .`.
+Update behavior notes and compatibility evidence. Save a semantic jj change with its own bookmark.
+Then select the next demonstrated requirement without waiting for another confirmation.
+
+Stop for missing access, a necessary product decision, or a destructive operation that needs approval.
+Do not replace unsupported hardware with game-specific patches or silent register stubs.
+Keep external ROMs, firmware, implementation source, and assets outside version control.