feat(cartridge): add 32 KiB SRAM save memory

Select it with --save-type sram (SaveDevice::Sram). The array is byte-wide,
starts erased, and repeats every 32 KiB through the 0x0e00xxxx save window.
A CPU byte store is staged and committed when its step completes, so failed
steps roll back and reads later in the step see the byte. Unlike Flash, SRAM
can be read from ROM code. Halfword/word access and DMA stay diagnostic until
the 8-bit bus behavior for those widths is verified.

SRAM shares the save-image API, host save files, and save states. Its device
code is new, so existing save states keep loading.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU

crates/desktop/tests/save_cli.rs

@@ -224,3 +224,33 @@ fn native_rom_window_save_reloads_in_another_process() {
         .to_string_lossy()
         .contains(".bak.")));
 }
+
+#[test]
+fn sram_bytes_written_and_read_from_rom_code_persist_across_processes() {
+    let f = Fixture::new();
+    let mut code = Vec::new();
+    load(&mut code, 0, 0x0e00_0000);
+    code.extend([0xe3a0_105a, 0xe5c0_1000]); // MOV r1,#0x5a; STRB r1,[r0]
+    load(&mut code, 0, 0x0e00_8001); // Mirror of offset 1.
+    code.extend([0xe3a0_103c, 0xe5c0_1000, 0xeaff_fffe]); // MOV r1,#0x3c; STRB; loop
+    let write = f.rom("sram-writer.gba", &code);
+    let save = f.save();
+    let output = run(&write, &save, "sram");
+    assert!(output.status.success(), "{}", text(&output));
+    assert!(text(&output).contains("Save persisted on clean exit"));
+    let image = fs::read(&save).unwrap();
+    assert_eq!(image.len(), 32768);
+    assert_eq!(&image[..2], &[0x5a, 0x3c]);
+    assert!(image[2..].iter().all(|&v| v == 255));
+
+    // Unlike Flash, SRAM reads need no work-RAM code.
+    let mut code = Vec::new();
+    load(&mut code, 3, 0x0e00_0000);
+    code.extend([0xe5d3_5000, 0xe5d3_6001, 0xeaff_fffe]); // LDRB r5,[r3]; LDRB r6,[r3,#1]
+    let read = f.rom("sram-reader.gba", &code);
+    let output = run(&read, &save, "sram");
+    assert!(output.status.success(), "{}", text(&output));
+    assert!(text(&output).contains("r5=0x0000005a"), "{}", text(&output));
+    assert!(text(&output).contains("r6=0x0000003c"), "{}", text(&output));
+    assert!(text(&output).contains("Save unchanged"));
+}