xenia-hid + xenia-debugger: gamepad serializer; debugger fast-skip hook

xenia-hid grows a guest-facing X_INPUT_GAMEPAD writer (big-endian on
the wire, host-neutral GamepadState in memory) so XamInputGetState in
the kernel and the UI input thread share one POD snapshot type. Adds
the GUIDE button flag.

xenia-debugger gains Debugger::wants_hooks(), a single-branch fast
path the hot interpreter loop checks to skip the pre_step/post_step
HashMap+match work when the debugger is in cold-run mode (no bps, no
trace, StepMode::Run, not paused). Part of the Tier-3 perf landing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/xenia-debugger/src/lib.rs

@@ -42,15 +42,30 @@ impl Debugger {
         }
     }
 
+    /// Tier-3 perf: single branch that the hot interpreter loop checks
+    /// before dispatching to [`pre_step`]/[`post_step`]. When the
+    /// debugger is in "cold run" mode (not paused, no breakpoints,
+    /// `StepMode::Run`, in-memory trace off), both hooks become dead
+    /// code and we can skip the HashMap lookup + step-mode match + Vec
+    /// maintenance entirely. The compiler reliably branch-predicts the
+    /// stable branch direction across millions of instructions.
+    #[inline]
+    pub fn wants_hooks(&self) -> bool {
+        self.trace_enabled
+            || self.paused
+            || self.break_pending
+            || !matches!(self.step_mode, StepMode::Run)
+            || !self.breakpoints.is_empty()
+    }
+
     /// Called before each instruction executes.
     pub fn pre_step(&mut self, ctx: &PpcContext, _mem: &dyn MemoryAccess) {
         // Check breakpoints
-        if let Some(bp) = self.breakpoints.get(&ctx.pc) {
-            if bp.enabled {
+        if let Some(bp) = self.breakpoints.get(&ctx.pc)
+            && bp.enabled {
                 self.break_pending = true;
                 tracing::info!("Breakpoint hit at {:#010x}", ctx.pc);
             }
-        }
     }
 
     /// Called after each instruction executes.