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/tool-direct-select.md

@@ -21,8 +21,8 @@ counterpart to Select, which moves the whole layer.
   By default both handles move symmetrically (smooth point).
 - **Alt+drag** a handle to break symmetry — corner point with
   independent handles per side.
-- **Click an anchor without dragging, then Alt+click** to flip
-  between smooth point and corner point.
+- **Alt+click an anchor** toggles between smooth point and corner
+  point.
 
 ## Multi-anchor edits
 
@@ -36,11 +36,13 @@ counterpart to Select, which moves the whole layer.
 
 ## Segment edits
 
-- **Drag a segment** between two anchors to bow it without moving
-  the anchors themselves (curve-direct-edit).
-- **Click an endpoint** when the path is open to continue drawing
-  from it (auto-flips you to the Pen tool with the path's last
-  anchor pre-selected).
+- **Drag a segment** between two anchors to bow it (curve-direct-
+  edit) without moving the anchors themselves.
+- **Alt+click a segment** to insert a new anchor at the click point
+  and drag it.
+- To continue drawing from an open endpoint, switch to the **Pen
+  tool** and click the endpoint — Direct Selection alone won't
+  extend the path.
 
 ## Tips