docs: audit pass — fix wrong shortcut + behaviour claims

Swept the help pages against the actual code paths and removed /
corrected claims that didn't ship.

vector-tools.md
- Removed: spacebar temp-pan, Alt duplicate-as-you-drag, generic
  Shift-constrain.
- Added: verified shortcuts (Cmd+Z/Y/D/C/X/V/G, Cmd+]/[ with
  Shift modifier, Cmd+J / Cmd+Alt+J in Pen edit mode,
  Delete/Backspace for layers + anchors).

tool-select.md
- Removed Cmd+A (not wired), Alt-drag-duplicate (not wired),
  Shift-constrain-axis drag (not wired), Cmd-click stack-walk
  (not wired). Kept the verified marquee + multi-select.

tool-direct-select.md
- Re-wrote anchor / handle interactions to match the actual hit-
  test branches (Alt-click anchor toggles kind in one click; Alt-
  drag a handle breaks symmetry; Alt-click segment inserts an
  anchor; plain drag of a segment bows it). Removed the "click an
  endpoint to continue drawing" claim — that's a Pen-only flow.

tool-hand.md
- Removed every fictional zoom shortcut (Cmd+wheel, Cmd+0,
  Cmd+= / Cmd+-). Documented the real zoom controls (−/+/Fit
  buttons + dropdown in the canvas toolbar). Spacebar reposition
  scoped correctly to the Pen tool only.

tool-pen.md
- Clarified that the Pen options pane has its own Fill / Stroke
  inputs (separate from any "Stroke panel" that doesn't actually
  exist).

tool-pencil.md
- Removed every claim that wasn't true: smoothing slider, Alt-drag
  append, auto-close. Pencil writes a plain polyline (M…L L L…),
  one layer per stroke. Tool options pane has only colour + width.

tool-shape.md
- Filled in the missing primitives (rounded-rect / circle /
  triangle — total 8, not 5). Removed Shift / Alt drag modifiers
  that aren't checked in startShapeDrag / commitShapeFromDrag.
  Added the small-drag → default-size fallback that the code does
  do.

tool-text.md
- Replaced the "drag a rectangle to size" claim with the actual
  click-to-place behaviour (placeTextWithTool drops a fixed-size
  box at the cursor + auto-flips to Select with the box focused).
  Softened the Cmd+B/I/U claims to "browser native rich-text
  shortcuts" since they're not wired in the app code.

tool-eyedropper.md
- Replaced the fictional "inline ⊙ buttons next to colour fields"
  with the real OS-colour-picker eyedropper that Chrome's native
  <input type="color"> shows.

panel-layers.md
- Removed Cmd+A select-all (not wired). Added Shift modifier to
  Cmd+]/[ for bring-to-front / send-to-back. Clarified that
  position units are inches in print modes, pixels in Custom Art
  / Slide / Deck (matches the unit toggle behaviour).

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/help/panel-layers.md

@@ -40,11 +40,14 @@ Each row in the layer list shows:
 - **Drag** rows to reorder. Z-order on the canvas follows the
   list order (top of the list = top of the stack).
 - **Cmd/Ctrl + ]** / **Cmd/Ctrl + [** sends selected layers
-  forward / backward.
-- **Shift+click** to multi-select; then drag a contiguous block.
+  forward / backward. Add **Shift** for bring-to-front / send-to-
+  back.
+- **Shift+click** (or Cmd / Ctrl + click) to multi-select rows.
 - **Group** — select multiple rows + click the Group button (or
-  Cmd/Ctrl + G) to collapse them under a single expandable row.
-  Groups have their own visibility / lock toggles that cascade.
+  press **Cmd/Ctrl + G**) to collapse them under a single
+  expandable row. Groups have their own visibility / lock toggles
+  that cascade.
+- **Ungroup** — click an active group's Ungroup button.
 
 ## Layer options (the per-layer right-side pane)
 
@@ -52,8 +55,9 @@ Clicking a layer in the list (or selecting it on the canvas)
 populates a contextual options block in the right-side floating
 pane. Common controls:
 
-- **Position** — x / y / w / h in inches (or px in screen-DPI
-  modes). Aspect-ratio lock keeps width + height proportional.
+- **Position** — x / y / w / h in canvas units (inches for print
+  modes; pixels for Custom Art / Slide / Deck). The 🔒 button
+  toggles aspect-ratio lock so changing w updates h proportionally.
 - **Rotation** — 0–360°.
 - **Opacity** — 0–1.
 - **Fit** — for image layers: cover / contain / fill / none.