Prune uploaded and pasted graphs correctly

[Ianyliu]

Summary

• retain each tab's original source, render options, and presentation state
• re-render uploaded and pasted DOT through /api/graphs/render when pruning
• keep default Terraform-project pruning on its compatibility endpoints
• make upstream/downstream traversal cycle-safe
• preserve an isolated selected node
• add backend and Node browser-state regressions for cycles, multiple tabs, uploads, pastes, and option retention
• document selection followed by the red prune action

Root Cause

Uploaded and pasted tabs kept their initial SVG and JSON objects in closure arguments. The prune handler requested a focused graph, but then immediately overwrote that response with the stale upload objects. The graph traversal also queued cyclic nodes indefinitely and only retained a selected node when it had an edge.

User Impact

Prune now operates on the graph visible in the selected tab—not the mounted Terraform project or an earlier rendering. Cyclic graphs terminate safely, isolated resources remain visible, and separate tabs cannot overwrite each other's source or options.

Validation

• /tmp/blast-radius-plan-venv/bin/python -m pytest -q — 21 passed, 7 subtests passed
• node --test tests/frontend/*.test.js — passed
• node --check blastradius/server/static/js/blast-radius.js
• real cyclic DOT render/refocus through the API — HTTP 200, two nodes and two edges
• git diff --check main...HEAD
• Windows wheel workflow now runs the Node browser-state regression

Compatibility

Default project graphs continue to use /graph.svg and /graph.json. Uploaded and pasted graphs use the render API introduced for #45. Existing tab, search, selection, and prune controls retain their behavior and presentation.

Fixes #5

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