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

shot_input.sh

@@ -71,6 +71,23 @@ for act in "$@"; do
             xd mouseup 1;           sleep 0.1
             xd keyup alt;           sleep 0.15
             ;;
+        wheel)
+            # "wheel X Y N [mod]" — N wheel clicks at (X,Y); N<0 scrolls down.
+            # X11 maps wheel up/down to buttons 4/5. Each click needs its own
+            # frame, same edge-detect reason as the click helper above.
+            xd mousemove "$2" "$3"; sleep 0.1
+            _n="$4"; _btn=4
+            if [ "$_n" -lt 0 ]; then _btn=5; _n=$(( -_n )); fi
+            _mod="${5:-}"
+            [ -n "$_mod" ] && { xd keydown "$_mod"; sleep 0.05; }
+            _i=0
+            while [ "$_i" -lt "$_n" ]; do
+                xd click "$_btn"; sleep 0.05
+                _i=$(( _i + 1 ))
+            done
+            [ -n "$_mod" ] && { xd keyup "$_mod"; sleep 0.05; }
+            sleep 0.15
+            ;;
         *) xd "$@"; sleep 0.15 ;;
     esac
 done