Add diatonic chord entry and velocity control

Chords are built by stacking scale thirds rather than from a quality table,
so the chord follows the degree automatically -- in C major, Alt+3 on C gives
C major, on D gives D minor, on B gives B diminished. Toggling delegates to
toggleCell, so chord tones inherit hold-mode joining, splitting and trimming
unchanged.

Velocity keys act on the note under the cursor if there is one, and on the
default for new notes otherwise, announcing which. That avoids a mode switch
between setting a level to draw at and shaping dynamics after the fact.

Also fixes install.ps1 failing to resolve its own directory: $PSScriptRoot is
not reliably populated during parameter binding, so it is resolved in the body.

Tests cover chord quality per scale degree and range clamping.

README.md

@@ -72,6 +72,9 @@ 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 |
+| `Alt+3` | Toggle diatonic triad on cursor pitch | — |
+| `Alt+7` | Toggle diatonic seventh chord | — |
+| `Alt+Shift+Up` / `Alt+Shift+Down` | Velocity up / down | — |
 | `Alt+G` | **Toggle grid mode on/off** | — |
 | `Alt+H` | Toggle hold mode | — |
 | `Alt+A` | Repeat current cell (speak + sound) | — |
@@ -112,6 +115,42 @@ it into one sustained note. Toggling a cell off in the middle of a sustained
 run splits it in two; at either end, it shortens it. So you can draw a long
 note by arrowing right and pressing Enter across several cells.
 
+## Chords
+
+`Alt+3` stamps a **diatonic triad** on the cursor pitch; `Alt+7` a seventh
+chord. Both are built by stacking scale thirds, so the quality follows the
+degree automatically — in C major, `Alt+3` on C gives C major, on D gives D
+minor, on B gives B diminished. There is no chord-quality table to configure
+and no way to land on a chord outside the key.
+
+If every tone of the chord is already in the cell, the keystroke removes them
+all; otherwise it fills in whatever is missing. Chord tones obey hold mode,
+splitting and trimming exactly like single notes.
+
+You can also build chords by hand at any time — arrow to a pitch, press Enter,
+arrow to another, press Enter. The cursor never moves horizontally when you
+toggle, so stacking notes in one cell has always worked. `Alt+3` and `Alt+7`
+are just the one-keystroke version.
+
+In the chromatic scale, "stacking thirds" degenerates into stacked whole
+tones. Predictable, but not musically useful — pick a real scale first.
+
+## Velocity
+
+`Alt+Shift+Up` and `Alt+Shift+Down` change velocity in steps of 8.
+
+The key does one of two things depending on where you are, and says which:
+
+- **On a note** — changes that note's velocity: *"E5 velocity 104"*, and
+  replays it at the new level so you can hear the change.
+- **On an empty cell** — changes the default for newly inserted notes:
+  *"Default velocity 104"*.
+
+So you can set a level and then draw at it, or draw first and shape the
+dynamics afterwards, without a mode switch either way.
+
+Set `velstep` in `ExtState` if steps of 8 are too coarse or too fine.
+
 ## Scales
 
 `Alt+S` prompts for a root (`C`, `F#`, `Bb`…) and a scale number. Up and down
@@ -145,6 +184,7 @@ In `ExtState` section `midigrid`:
 | Key | Default | Meaning |
 |---|---|---|
 | `vel` | 96 | Velocity of inserted notes |
+| `velstep` | 8 | Velocity change per keypress |
 | `chan` | 0 | MIDI channel (0-based) |
 | `previewdur` | 0.4 | Preview note length, seconds |