fix: restore waveform symmetry about the midline, cut deck_values' link to the bank model, and unit-test the column arithmetic

The waveform column's vertical extents move to pure component_geometry so the shared
primitive stops being untested; PlaySeconds hoists into a header-only play_seconds target.

src/core/instrument/ui/deck_values.h

@@ -2,13 +2,13 @@
 // normalized 0..1 a knob shows, the write back into the stored seconds/fractions/positions, the
 // double-click reset, and the ms time-constant formatter. Split from the editor shell so the
 // whole domain map is provable without a host; deck_groups owns WHICH controls exist, this owns
-// what each one's value MEANS. Links the map layer because PlaySeconds is what a deck edits.
+// what each one's value MEANS.
 
 #pragma once
 
 #include <cstddef>
 
-#include "core/instrument/map/sample_map.h"        // PlaySeconds (the deck's edit target)
+#include "core/instrument/map/play_seconds.h"      // PlaySeconds (the deck's edit target)
 #include "core/instrument/ui/deck_groups.h"        // DeckParam
 #include "core/instrument/ui/envelope_overlay.h"   // kGateStageMaxSeconds
 
@@ -39,9 +39,11 @@ double deckParamNorm(DeckParam id, const PlaySeconds& play);
 void setDeckParam(DeckParam id, PlaySeconds& play, double value, int segment);
 
 // Resets `id` to its default. The default IS what a fresh PlaySeconds carries, so there is no
-// second table of defaults to drift from the real one. Exact for every shipped default: they
-// are all 0, 1, or the curve neutral, and the seconds ceiling is a power of two, so the
-// norm round trip loses nothing. For knob-valued controls — a toggle has no reset gesture.
+// second table of defaults to drift from the real one. It arrives via the norm round trip, so
+// landing EXACTLY on a stage time (0.003 s attack, 0.060 s release) depends on
+// kEnvTimeMaxSeconds being a power of two — x/2^n*2^n is lossless, an arbitrary ceiling is not.
+// Move that ceiling off a power of two and a reset lands a mantissa bit off its own default.
+// For knob-valued controls — a toggle has no reset gesture.
 void resetDeckParam(DeckParam id, PlaySeconds& play);
 
 // A time constant as MILLISECONDS, e.g. "12 ms". Never switches to seconds: the editor reads in