Fix #483: Shift-drag marquees toggle the selection; the Lasso's Alt keeps the rest

[bflatastic]

A Shift-drag marquee only ever added to the selection; as in Illustrator it now toggles what it reaches, so selected objects (or anchors) can be taken out with a marquee.

• Selection tool: the Shift marquee runs select.toggle, which now takes {ids} too (each one toggles); a Shift-click on empty canvas still leaves the selection alone.
• Direct Selection / Group Selection: the Shift marquee toggles the anchors inside it (select.anchorsMany mode toggle); a path left with none leaves the selection, one with all of them is selected whole again.
• select.anchorsMany takes mode: "set"|"add"|"toggle"|"subtract" (add: true still means add; an unknown mode is a parameter error). Adding anchors of a path selected whole keeps it whole, and references outside the path are ignored.
• Lasso: Shift adds and Alt subtracts through the same command, so an Alt lasso no longer drops selected type, groups and other non-path objects from the selection.
• Docs (docs/mcp.md, with a pointer_gesture example), ROADMAP (Last updated, Selection bullet), command-sweep cases for the new modes.

Illustrator reference: Shift-click or Shift-drag around objects adds or removes them from the selection (Select objects); the Lasso adds with Shift and subtracts with Alt/Option (Select anchor points).

Checks

• Rebased on d9fc3b6505bb185c7ecce05ab4b60b86da12822f (origin/main).
• cargo fmt --all --check; cargo clippy -p vectorcraft-tools -p vectorcraft-engine -p vectorcraft-mcp --all-targets -- -D warnings: clean.
• cargo test -p vectorcraft-tools -p vectorcraft-engine -p vectorcraft-mcp: all pass (tools 242, engine lib 1358, mcp lib 57, command sweep). New: select::shift_marquee_toggles, direct::shift_marquee_toggles_anchors, engine selection_shift_marquee_toggles_objects, direct_selection_shift_marquee_toggles_anchors (Group Selection too), lasso_shift_adds_and_alt_subtracts, MCP shift_marquee_gesture_toggles_the_selection. The three engine tests fail on main (Shift marquee added b instead of removing it; Direct Selection added; Alt lasso dropped the type).
• cargo xtask layers, cargo xtask brands, cargo xtask wasm: OK; cargo check -p vectorcraft-engine --target x86_64-unknown-linux-gnu / aarch64-apple-darwin: OK.
• Live (desktop app, control port ui.pointer + ui.screenshot): with two of three squares selected, a Shift marquee over the second and third leaves the first and third selected; a Direct Selection Shift marquee across the top edges toggles those anchors (first square partly selected, second joins with its top anchors).
• MCP (vectorcraft-cli mcp): the documented pointer_gesture gives [a, c] from [a, b]; an Alt lasso keeps the selection; select.anchorsMany {mode: "bogus"} is a parameter error.

🤖 Generated with Claude Code

[bflatastic]

Full cargo xtask ci passed on the integrated stack; merging.