Route delete / undo / connect crossing paths through boundaryProxyView.survivingProxies

[CuriouslyCory]

What to build

With the boundaryProxyView module in place (#145), replace the inline re-implementations of "which boundary proxies survive when these Connections are removed" that currently live independently in three island paths: component/connection delete, undo, and connect. Today the delete path re-derives this inline while undo already calls an extracted helper — they can drift.

Route all three through boundaryProxyView.survivingProxies so the rule lives in one tested place. This is where the locality win lands: orphaned-proxy cleanup, coalesced-node survival, and proxy re-add on undo all resolve through the same interface.

Sequenced after the cross-scope write migration (#148) because both touch the same handlers (commitDeleteConnection, commitConnect, the undo path); doing this second avoids reworking the same code twice.

Source: architecture review, candidate 02b.

Acceptance criteria

• Delete, undo, and connect paths compute surviving / orphaned proxies via boundaryProxyView.survivingProxies — no inline re-derivations remain
• The previously-inline delete-path logic and the undo readdCrossScope helper resolve through the one module interface
• Behavior is unchanged: deleting one of several crossing Connections keeps the coalesced proxy; deleting the last removes it; undo restores proxies correctly — verified in the running app
• pnpm check and pnpm test pass

Blocked by

• Extract the boundaryProxyView module #145
• Migrate cross-scope canvas writes onto the optimisticWrite seam #148