feat: scale time zoom by duration, decouple the two axes

The minimum visible time window was a flat 2% of the file, but view.start
and view.end are normalized to the whole signal — so the achievable time
resolution scaled with file length. A 30-minute recording could never show
less than a 36-second span, while a 30-second one reached 0.6 s. Long
captures were effectively unreadable at the sample level no matter how far
you scrolled.

Derive the floor from the STFT hop instead (MinTimeViewWidth): segments sit
fftSize/HOP_RATIO samples apart, so the real limit is the point where only
a handful of segments span the viewport and further zoom would interpolate
rather than reveal. The floor is now a constant ~43 ms at 48 kHz/1024
regardless of duration — an 844x improvement on a 30-minute file, and it
tightens further with a smaller FFT. Guards cover the unloaded (sampleRate
0) and shorter-than-the-floor cases.

Zooming is also no longer forced to move both axes together. The bare wheel
keeps the existing coupled behaviour; Shift+wheel is time-only and
Ctrl+wheel frequency-only, so a long capture can be stretched along time
without collapsing the frequency range to match.

Time-axis labels now pick their precision from the span between adjacent
ticks (1 to 4 decimals, and m:ss.sss past a minute). At the spans this
change makes reachable the old fixed "%.1fs" printed the same value in
every slot, which read as a frozen axis.

Adds a `wheel X Y N [mod]` action to shot_input.sh for exercising zoom
headlessly, and known_bugs.md for behaviour that is unspecified rather
than broken.

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

README.md

@@ -129,7 +129,9 @@ or pressing **O** for the file browser. Try the bundled sample:
 | Input | Action |
 |-------|--------|
 | **O** | Open file browser |
-| **Mouse wheel** | Zoom time/frequency |
+| **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 |
 | **Space** | Play / stop the selected region |
@@ -229,6 +231,13 @@ 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.
+- **Time zoom limit** — the tightest visible window is derived from the STFT hop
+  (`fftSize / HOP_RATIO` samples), not from a fixed fraction of the file, so time
+  resolution does not degrade as files get longer: a 30-minute recording zooms in
+  just as far as a 30-second one. At 48 kHz / 2048-point FFT the floor is ~85 ms
+  across the viewport; a smaller FFT zooms correspondingly tighter. Past that
+  point there are no further STFT segments to show, so the view would only
+  interpolate.
 - **Playback / WAV export** share one processing path: the selected time span,
   FFT-bandpassed to the selected frequency box, peak-normalised.
 - **mLnL parsing** — walks the WAV's RIFF chunks for the four-CC `mLnL` chunk
@@ -253,3 +262,6 @@ src/
 See [`raylib_for_desktop_applications.md`](raylib_for_desktop_applications.md)
 for the performance / idle-CPU lessons behind the desktop build, and
 [`AGENTS.md`](AGENTS.md) for the headless-testing playbook.
+
+Known rough edges — behaviour that is unspecified or awkward rather than simply
+broken — are tracked in [`known_bugs.md`](known_bugs.md).