Add whole-take time shifting

Ctrl+Shift+Left/Right move every note in the take by one grid cell, for
clips that landed off the beat.

Since the whole take moves together, notes cannot collide with each other;
the hazard is the item boundary. Notes pushed past either end stop sounding
but are not deleted, so the operation is reversible -- shifting back restores
them. The plain keys still refuse when that would happen and report the count,
because a clip that has silently lost its first bar is a bad thing to discover
later. Ctrl+Alt+Shift forces it and names the consequence.

Positions are converted through QN rather than offset in raw ticks, so shifts
stay musically correct across tempo changes.

README.md

@@ -72,6 +72,8 @@ actions by hand in Actions → Show action list. That always works.
 | `Left` / `Right` | Cursor back / forward one grid cell | OSARA previous / next chord |
 | `Up` / `Down` | Pitch up / down one scale degree | OSARA higher / lower note in chord |
 | `Enter` | Toggle note at cursor cell | Edit: Event properties |
+| `Ctrl+Shift+Left` / `Ctrl+Shift+Right` | Shift **all** notes one cell, refusing if any would leave the item | — |
+| `Ctrl+Alt+Shift+Left` / `Ctrl+Alt+Shift+Right` | Same, but shift anyway | — |
 | `L` / `Shift+L` | Lengthen / shorten note by one cell | Edit: Lengthen / shorten notes one grid unit |
 | `Alt+J` | Join a run of repeated notes into one | — |
 | `Alt+3` | Toggle diatonic triad on cursor pitch | — |
@@ -147,6 +149,27 @@ survive. They also need no mode: `L` behaves identically whether hold is on or
 off, so there is no state to set up beforehand and none to remember to turn
 off afterwards.
 
+## Shifting notes in time
+
+If a whole clip landed a couple of cells off the beat, `Ctrl+Shift+Left` and
+`Ctrl+Shift+Right` move **every note in the take** by one grid cell.
+
+Because the entire take moves together, notes can never collide with each
+other. What is at stake is the **item boundary**: notes pushed past either end
+stop sounding. They are *not* deleted — shifting back the other way brings
+them straight back — but a clip that has quietly lost its first bar is not
+something you want to discover two hours later.
+
+So the plain keys **refuse** the shift if any note would end up outside, and
+say how many: *"Not shifted left, 3 notes would fall outside the item. Add Alt
+to force."*
+
+Adding **Alt** (`Ctrl+Alt+Shift+Left` / `Right`) does it anyway, reporting the
+consequence: *"Shifted left, 3 notes now outside the item and silent."*
+
+Positions are converted through quarter-notes rather than offset in raw ticks,
+so a shift stays musically correct across tempo changes.
+
 ## Chords
 
 `Alt+3` stamps a **diatonic triad** on the cursor pitch; `Alt+7` a seventh