app: the graph view

Space swaps the code view for the function's basic blocks, IDA's own key.
Boxes hold the SAME Head rows the listing renders, so IDA's colour tags,
the word highlight, the execution trail and every editing verb work inside
them for free. Nothing is pre-painted: each screen row is composed on
demand from the edge index plus whichever boxes cover it, so cost tracks
the viewport and not the graph.

z cycles three zoom levels, m toggles a minimap, J/K walk edges, and the
mode is sticky -- following a call lands in the callee's graph. Above 400
blocks it declines and says so, because nothing readable comes out at that
size. Edges follow IDA's colours, and the ones touching the block under
the cursor are brightened.

Adding a value to _active means every consumer has to learn it: the one
that was missed (_active_code_view returning None) crashed the app the
first time a prompt closed in graph mode. A stale async load is also
guarded now -- without it, a rename's queued rebuild landed later and
dragged the user back into a graph they had already dismissed.

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