feat(power): implement keypad-wake STOP and BIOS Stop service

Separate Running, Halt, and Stop states. Complete the entering instruction before freezing CPU, DMA, timers, display, and externally supplied clock advances. Wake STOP through the live enabled keypad condition without creating a clock-off IF request.

Implement original ARM SWI 0x03, preserve caller state, and expose zero-cycle StopIdle plus stopped frame-runner results. Retain DMA state, timer phase, and captured frames across sleep. Document keypad-only wake and unverified oscillator/edge timing.

Add core, BIOS, and demo regression tests; preserve BIOS-only access and failed-store isolation. Validated on Darwin arm64 with debug/release workspace and core/demo suites, Clippy, rustdoc, ten native window smoke tests, and terminal demos.

crates/gba-core/src/io.rs

@@ -123,6 +123,14 @@ impl Timer {
     }
 }
 
+#[derive(Debug, Default, Clone, Copy, PartialEq, Eq)]
+enum PowerState {
+    #[default]
+    Running,
+    Halt,
+    Stop,
+}
+
 #[derive(Debug, Default, Clone, PartialEq, Eq)]
 pub(crate) struct Io {
     timers: [Timer; 4],
@@ -142,7 +150,7 @@ pub(crate) struct Io {
     display: Display,
     dma: Dma,
     postflg: bool,
-    halted: bool,
+    power: PowerState,
 }
 
 impl Io {
@@ -264,9 +272,12 @@ impl Io {
             IME if address & 1 == 0 => self.master_enable = value & 1 != 0,
             POSTFLG if address == POSTFLG => self.postflg = value & 1 != 0,
             POSTFLG => {
-                // The bus has already rejected STOP and enforced CPU write access.
-                debug_assert_eq!(value & 0x80, 0);
-                self.halted = true;
+                // The bus has enforced BIOS-only CPU access. Low seven bits are ignored.
+                self.power = if value & 0x80 == 0 {
+                    PowerState::Halt
+                } else {
+                    PowerState::Stop
+                };
             }
             _ => {} // Unused I/O bytes ignore writes.
         }
@@ -274,13 +285,23 @@ impl Io {
     }
 
     pub(crate) fn halted(&self) -> bool {
-        self.halted
+        self.power == PowerState::Halt
+    }
+
+    pub(crate) fn stopped(&self) -> bool {
+        self.power == PowerState::Stop
     }
 
     fn wake_if_requested(&mut self) {
-        // Wake-up ignores IME and CPSR.I. It does not acknowledge IF.
-        if self.enable & self.pending != 0 {
-            self.halted = false;
+        // Both wake paths ignore IME and CPSR.I and leave IF unchanged.
+        // STOP uses the live keypad condition, not a stale IF latch.
+        let wake = match self.power {
+            PowerState::Running => false,
+            PowerState::Halt => self.enable & self.pending != 0,
+            PowerState::Stop => self.enable & (1 << 12) != 0 && self.keypad.wake_condition(),
+        };
+        if wake {
+            self.power = PowerState::Running;
         }
     }
 
@@ -298,7 +319,10 @@ impl Io {
     }
 
     pub(crate) fn set_buttons(&mut self, buttons: Buttons) {
-        if self.keypad.set_buttons(buttons) {
+        if self.stopped() {
+            // System clock is off: update wake inputs without latching IF/history.
+            self.keypad.update_stopped_buttons(buttons);
+        } else if self.keypad.set_buttons(buttons) {
             self.pending |= 1 << 12;
         }
         self.wake_if_requested();
@@ -318,8 +342,12 @@ impl Io {
                 touched = true;
             }
         }
-        if touched && self.keypad.write_control(control) {
-            self.pending |= 1 << 12;
+        if touched {
+            if self.stopped() {
+                self.keypad.update_control(control);
+            } else if self.keypad.write_control(control) {
+                self.pending |= 1 << 12;
+            }
         }
         self.wake_if_requested();
     }