Clamp auto-size to the supported grid range - #16

The gridClamped flag was written from both auto-size and handleAddTool and
cleared from neither of the paths that matter. Deleting the last tool made
the auto-size effect return early, so the banner stayed on screen over an
empty bin; adding a small tool to an already-clamped 10u bin computed
max(10, small) == 10, which is not over the limit, so it wrongly cleared a
banner that was still true.

Compute it instead from the tool bounds, where it cannot go stale. That
needed the "grid units to fit this span" maths in one place, so it moves to
lib/binGrid.ts along with the bounds walk both call sites had inlined.

The banner now also says that the overflowing geometry is cut off, which is
what actually happens once the grid stops growing.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

The banner reads from the union bounds of every placed tool, so "a tool needs
more room" is wrong whenever two tools together overflow a bin that either
would fit alone, and "move that tool into a bin of its own" is impossible
advice when one tool really is longer than 420mm. Say both things.

It also fired with auto-size off, describing a grid that had stopped growing
when nothing was growing it. Gate on autoSize.

handleAddTool dropped a tool with no points rather than adding it un-centred.
Unreachable for a traced tool, but silently discarding what the user asked for
is the wrong failure.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>