Pen: holding Space while dragging out a point moves it

[theironchef]

Fixes #313. Stacked on #310: the first commit here is #310's. Once that one merges, this PR shows only its own commit.

Problem: the Pen never read Space during a drag. Holding it while dragging out a point left the anchor where the button went down, and its handles swung round it. In Illustrator, holding the spacebar while the button is still down moves the anchor being placed, with its handles. That's the Pen's counterpart of #302 / #304 for the shape tools.

Change (crates/tools/src/pen.rs)

• PenTool remembers the pointer at the last button-down or drag (last).
• In the Drag arm, while Space is held (and the drag isn't closing the path), the anchor moves as far as the pointer did. The outgoing handle is the pointer and the incoming one mirrors it, so both move with it.
• The incoming handle Alt kept (Pen: Alt pressed mid-drag keeps the curve into the new anchor #310) moves along too.
• Releasing Space goes back to shaping the handles round the anchor's new place.
• This covers the first anchor of a new path (path.create) and an anchor added to a path (path.appendAnchor).
• The canvas already passes Space to the tool during a drag (canvas::mods(m, space)), so nothing else changes.
• The module doc says so.

Tests

• New pen::tests::space_moves_the_anchor_being_placed:
  • for a new path's first anchor, Space held moves the point, handles and all, and once released the drag shapes the handles round the new place;
  • for an anchor added to an open path, with Alt and Space held, the anchor and the kept incoming handle move together.
• With the Space branch disabled, the test fails (the anchor stays at y = 420).
• cargo test -p vectorcraft-tools: 190 passed.
• cargo test -p vectorcraft-engine --lib pen: 59 passed.
• cargo clippy -p vectorcraft-tools --all-targets -- -D warnings (Rust 1.99): clean.
• cargo fmt --all -- --check: clean.
• Checked by hand in a release build on Windows with real mouse and keyboard input:
  • I clicked, pressed further on and dragged right, held Space and dragged 50 pt up, released Space and dragged on.
  • The point rode up with its handles, and then the drag shaped the handles again. The view didn't pan.
  • The committed path is M0.5 50.576 C0.5 50.576 20.53 20.303 100.652 0.5: the anchor at about (150, 100) instead of (150, 150), with its incoming handle mirroring the final outgoing one.

I didn't run the full cargo xtask ci (wasm step) here.

🤖 Generated with Claude Code

[bflatastic]

Thanks @theironchef, merged! Holding Space while dragging out a point now moves it, handles and all. I rebased it on main and added the ROADMAP entry in a commit on top.

Live check: Space held mid-drag moved the anchor to (300,60) with its handles at (240,60) / (360,60), and releasing Space went back to shaping the handles. Also checked over MCP. One follow-up you might like: Illustrator also lets Space reposition the closing anchor while closing a path.