UI: stale-graph banner when SQL is edited after analysis

[melonamin]

Split out from #17.

Problem

After the graph is built, editing SQL silently invalidates node spans — graph↔text navigation will jump to wrong offsets.

Proposal

• Track the parsed SQL digest alongside the graph.
• When editor content diverges, show a non-blocking banner: "Graph is out of sync with text — re-run analysis to enable navigation."
• Disable navigation affordances (◀, ▶, focus buttons) while stale.
• Clear the banner when (a) content returns to the parsed digest (undo), or (b) user re-runs analysis.

Acceptance

• Banner toggles on edit/undo.
• Navigation buttons disabled while stale.
• Playwright test covers edit → undo → edit → reparse cycle.

Independent of the model changes — can land in parallel.