fix(frontend): address PR #49 review comments

- ui_shell.rs: the "no debugger snapshot yet" message no longer
  claims a ROM-load reason it doesn't actually check -- debug tracks
  debugger_open, not ROM state (a snapshot builds fine for a blank
  core).
- debug_snapshot.rs: stop claiming vram_window_start is "controlled
  by the debugger UI's scroll position" -- no UI calls
  set_debug_vram_scroll yet, so the window is fixed today.
- Five stale `docs/frontend.md §open questions` references updated to
  `§Debugger overlay`, the section that actually documents this now.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: doublegate

crates/rustysnes-cart/src/board.rs

@@ -133,7 +133,7 @@ pub trait Board {
     /// The GSU register file (R0-R15 + SFR + PBR), for a Super FX board's debugger Cart panel.
     ///
     /// Default `None` — only [`crate::coproc::superfx::SuperFxBoard`] overrides this. A
-    /// read-only debug accessor, not a control surface (`docs/frontend.md` §open questions).
+    /// read-only debug accessor, not a control surface (`docs/frontend.md` §Debugger overlay).
     fn debug_gsu_state(&self) -> Option<([u16; 16], u16, u8)> {
         None
     }

crates/rustysnes-cart/src/coproc/gsu.rs

@@ -414,7 +414,7 @@ impl Gsu {
 
     // --- Debug-only read accessors (no side effects, unlike `read_register`'s memory-mapped
     // window which can have read-clear/latch behavior on some addresses). For the debugger
-    // overlay's Cart panel (`docs/frontend.md` §open questions). ------------------------------
+    // overlay's Cart panel (`docs/frontend.md` §Debugger overlay). ------------------------------
 
     /// The R0-R15 general-purpose register file (R15 is also the program counter).
     #[must_use]

crates/rustysnes-core/src/scheduler.rs

@@ -205,7 +205,7 @@ impl System {
     }
 
     /// The SA-1 second CPU's architectural register file, or `None` when no SA-1 cart is
-    /// installed. For the debugger overlay's Cart panel (`docs/frontend.md` §open questions).
+    /// installed. For the debugger overlay's Cart panel (`docs/frontend.md` §Debugger overlay).
     #[must_use]
     pub fn sa1_regs(&self) -> Option<Regs> {
         self.sa1_cpu.as_ref().map(|c| c.regs)

crates/rustysnes-frontend/src/app.rs

@@ -439,7 +439,7 @@ impl App {
                 rom_loaded: emu.rom_loaded(),
             };
             // Only build the debugger snapshot when the window is actually open — a real,
-            // avoidable per-frame cost otherwise (`docs/frontend.md` §open questions).
+            // avoidable per-frame cost otherwise (`docs/frontend.md` §Debugger overlay).
             let debug = active.shell.debugger_open.then(|| emu.debug_snapshot());
             drop(emu); // release the brief lock BEFORE the wgpu upload + egui pass
             (fb, dims, info, audio_samples, debug)

crates/rustysnes-frontend/src/debug_snapshot.rs

@@ -43,9 +43,10 @@ pub struct PpuSnapshot {
     /// The full 256-entry CGRAM palette (512 bytes — cheap to copy wholesale every frame, unlike
     /// VRAM's 64 KiB).
     pub cgram: [u16; 256],
-    /// A [`VRAM_WINDOW_LEN`]-word window of VRAM starting at `vram_window_start` (word address),
-    /// controlled by the debugger UI's scroll position — copying all 64 KiB every frame would be
-    /// real, avoidable per-frame cost for a window the user can only look at part of at once.
+    /// A [`VRAM_WINDOW_LEN`]-word window of VRAM starting at `vram_window_start` (word address) —
+    /// copying all 64 KiB every frame would be real, avoidable per-frame cost for a window the
+    /// user can only look at part of at once. `EmuCore::set_debug_vram_scroll` moves the window;
+    /// no UI control calls it yet (fixed at the window's start address today) — a follow-up.
     pub vram_window: [u16; VRAM_WINDOW_LEN],
     /// The word address `vram_window` starts at.
     pub vram_window_start: u16,

crates/rustysnes-frontend/src/ui_shell.rs

@@ -313,7 +313,9 @@ impl ShellState {
                 });
                 ui.separator();
                 let Some(debug) = debug else {
-                    ui.label("(no ROM loaded — nothing to inspect yet)");
+                    // `debug` tracks `debugger_open`, not ROM state (a snapshot builds fine for a
+                    // blank core) — don't claim a ROM-load reason that may not be why it's `None`.
+                    ui.label("(no debugger snapshot yet)");
                     return;
                 };
                 match self.panel {

crates/rustysnes-ppu/src/lib.rs

@@ -892,7 +892,7 @@ impl Ppu {
     }
 
     /// The current `BGMODE` ($2105) value (0..=7) — which of the 8 tile/priority layouts is
-    /// active. For the debugger overlay's PPU panel (`docs/frontend.md` §open questions).
+    /// active. For the debugger overlay's PPU panel (`docs/frontend.md` §Debugger overlay).
     #[must_use]
     pub const fn bg_mode(&self) -> u8 {
         self.io.bg_mode