feat: menubar + icon rail, minimap, Blender-style navigation

A pass over the whole interface, driven by using it on a 5.7-hour capture.

**Layout.** The 320px sidebar of labelled widgets is now a 46px icon rail —
one column of square buttons, sized so it costs the spectrogram as little
width as possible — plus a menubar for one-shot actions. Menu items are
defined by naming a keymap entry, so an item reuses that binding's action,
gate and shortcut label and the two can't drift; items grey out under
exactly the conditions that make the shortcut a no-op. Settings that need
more than an on/off (FFT size, colours/levels, annotation kinds) open as
popouts beside the rail rather than widening it. Every toggle has exactly
one home: nothing is reachable from both the rail and a menu.

Icons are drawn from raylib primitives rather than an atlas or font
glyphs — the bundled font has no symbol coverage, and vector shapes stay
crisp at any UI scale with no assets to ship.

**Navigation.** Left-drag now pans and Ctrl+drag box-selects, with Tab
swapping which is bare (Ctrl always means "the other one", so either mode
does both). Previously a bare left-drag did three different things
depending on invisible state, with no cursor feedback; the cursor now
reports the active gesture. Wheeling a scrollbar pans that axis, or zooms
it with Shift.

**Minimap.** Whole-file thumbnail in the top-right with the current view
drawn on it; click or drag to scrub, corner handle switches between two
sizes. Rendered once per size into a cached texture and rebuilt only when
its content changes — panning and zooming just move the rectangle drawn on
top. The reduction is strided (each thumbnail cell samples at most 8x8),
because reducing every segment x bin meant ~1 G reads per rebuild and a
visible hitch on every overlay toggle.

**Fixes found along the way:**
- The timeline lane mapped events across the whole file while the
  spectrogram above it showed a zoomed window, so the two only lined up at
  full zoom-out and an event's tick sat nowhere near its burst. It is also
  properly toggleable now: the old flag only grew an always-present lane.
- Clicks preferred the spectrogram over the minimap. Input handling runs
  ~900 lines before the draw pass that computed the minimap's rect, so a
  press there started a pan AND a scrub — two handlers writing app.view in
  one frame. Geometry queries that input depends on now live outside the
  draw pass.
- Repainting during the background fill re-ran the full synchrosqueeze
  every 0.5 s. Zoomed out that is ~0.5 G bin-visits with four trig calls
  each, twice a second, for minutes — while showing almost nothing new,
  since folded segments land in columns already drawn. The interval now
  scales with how much work a repaint actually costs.
- IsUserInteracting() sweeps 512 key codes and was called three times a
  frame; memoized per frame.
- Frequency labels were drawn at a fixed offset wider than their gutter and
  overflowed into the rail. They are measured and right-aligned now, with
  one format chosen per axis so the column doesn't mix "3k" with "2.3k".
- The horizontal scrollbar is pinned to the window bottom; it used to sit
  mid-layout competing with the scope's divider, and the scope covered it
  outright at larger window sizes.
- The scope starts hidden — the spectrogram is the primary view.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V8ZWfr5XZyyDttvkhJUgHN

README.md

@@ -126,17 +126,24 @@ or pressing **O** for the file browser. Try the bundled sample:
 
 ### Controls
 
+**Navigating.** A left-drag pans by default and **Ctrl+drag** draws a selection
+box; **Tab** (or the rail's pan/select icons) swaps which one is bare, and Ctrl
+always means "the other one", so either mode does both without switching back.
+Middle-drag always pans.
+
 | Input | Action |
 |-------|--------|
-| **O** | Open file browser |
+| **LMB drag** | Pan the view (**Ctrl+drag** to box-select) |
+| **Tab** | Swap pan / select mode |
+| **Middle-drag** / **Alt+drag** | Pan, regardless of mode |
 | **Mouse wheel** | Zoom both axes (preserves aspect ratio) |
 | **Shift+wheel** | Zoom the time axis only |
 | **Ctrl+wheel** | Zoom the frequency axis only |
-| **Alt+drag** / **middle-drag** | Pan the view |
-| **LMB drag** | Select a time + frequency region |
+| **Wheel on a scrollbar** | Pan that axis (**Shift** to zoom it) |
 | **Space** | Play / stop the selected region |
 | **Hover an annotation** | Tooltip with that frame's mLnL detail; lists **every** overlapping frame under the cursor |
 | **N** / **Shift+N** | Jump to the next / previous collision |
+| **O** | Open file browser |
 | **P** | Show / hide the waveform scope |
 | **M** | Marker / ruler tool |
 | **S** | Spectrum slice (PSD) |
@@ -148,8 +155,30 @@ or pressing **O** for the file browser. Try the bundled sample:
 | **F1** | About / help |
 | **Esc** | Clear selection / close dialog |
 
-Most controls are also available as buttons in the left sidebar (colormap, floor,
-dynamic range, annotation opacity, grid, …).
+### Layout
+
+A menubar across the top holds one-shot actions (**File** — open, export
+PNG/WAV; **View** — reset/zoom, hide the icon rail, fullscreen; **Annotations** —
+jump to next collision; **Help**). Everything that toggles lives on the rail
+instead, so no control has two homes. Menu items are defined by naming a keyboard shortcut, so an item and
+its key can never drift apart, and items grey out under exactly the conditions
+that make the shortcut a no-op.
+
+Down the left is a narrow **icon rail** — one column of square buttons, sized so
+it costs the spectrogram as little width as possible. Hover any icon for a
+tooltip. Left to right in function: play/stop and clear selection; pan/select
+mode; marker, spectrum slice, scope, grid, minimap; FFT size and colour/level
+popouts; annotations, collisions, and the timeline lane. The three settings
+popouts open beside the rail rather than widening it. `View → Hide icon rail`
+hands its width back to the spectrogram.
+
+The **minimap** (top-right, toggled from the rail) is a thumbnail of the whole
+capture with the current view drawn on it — click or drag anywhere on it to
+scrub. Annotation density runs along its bottom edge and collisions along its
+top. The corner handle switches between two sizes. It is rendered once into a
+texture and only rebuilt when its *content* changes (new file, colormap,
+overlays toggled); panning and zooming just move the rectangle drawn on top, so
+navigation costs nothing.
 
 ### Inspecting overlapping transmissions
 
@@ -254,6 +283,14 @@ paths.
   frequency resolution `sampleRate / fftSize` Hz per bin. Amplitude in dB.
 - **Axes** — X = time (s), Y = frequency (Hz, scaled to the file's Nyquist),
   colour = amplitude.
+- **Loading** — the STFT overview is computed in one blocking pass behind the
+  progress panel. It used to advance a fixed number of segments per frame, which
+  made loading frame-paced rather than compute-bound: the frame limiter, not the
+  FFT, set the speed, so a 478k-segment capture spent over a minute waiting
+  between frames. The tell was that backgrounding the window — which skips
+  presenting entirely — loaded the same file in seconds. Background work also
+  continues while the window is unfocused, so a long capture can be left to
+  finish behind another window.
 - **Long files** — two things keep cost tied to what's on screen rather than to
   total duration. The spectrogram image is built for the *visible* segment range
   (capped at 8192 px wide), so a multi-hour capture renders at all — an
@@ -285,8 +322,9 @@ paths.
 src/
   spectrogram.c        # entry point, main loop, CLI args, headless render
   stft.c / fft.c       # STFT + FFT
-  render.c             # spectrogram, annotations, tooltips, scope
-  ui.c                 # sidebar, file browser, buttons
+  render.c             # spectrogram, annotations, tooltips, minimap, scope
+  ui.c                 # menubar, icon rail + popouts, file browser
+  primitives.c         # waveform scope + its min/max envelope summary
   audio.c              # WAV load (ffmpeg fallback), bandpass, playback, WAV export
   mlnl.c / mlnl.h      # mLnL annotation chunk parser
   platform_*.c         # per-OS shims (linux / win32 / web)