feat(kb): graph navigability — focus + context map (implements #73)

[jordanrburger]

Implements the KB graph navigability feature per the spec + plan merged in #73, with the merge-time review notes applied where they override the plan.

What changed

Task 1 — engine (KBGraph.swift, KnowledgeBaseService.swift)

• KBGraph.topHubs(maxNodes:) — top nodes by degree (id-ascending tiebreak), only edges internal to the kept set; no-op under the cap.
• KBGraph.filtered(types:hideOrphans:minDegree:) — pure post-filter; always retains the center so a re-rooted view is never emptied.
• KnowledgeBaseService.hubGraph(maxNodes: 40) — the overview's default seed (fullGraph().topHubs).
• New KBGraphTransformTests suite (6 tests, TDD; fixtures use the neutral alex/priya/sam stand-ins).

Task 2 — KBMapView (new) embedded in KBOverviewView in place of the unbounded fullGraph() map

• Opens on the top-40 hubs; single-tap re-roots on the tapped node (localGraph(around:depth:2,maxNodes:40)); tapping the centred node again — or the breadcrumb "Open" button — opens it in the editor.
• Name search (≥2 chars, 8 matches) → focus; breadcrumb with ⌂ Hubs / ‹ Back / current center.

Task 3 — filters + honest captions

• Entity-type toggle chips (legend colours), "Hide orphans", min-links stepper.
• "showing N of M notes" caption whenever the map renders.

Review notes from #73 applied

1. Stale-focus fallback (spec, not the plan's no-reset): an effectiveFocus guard checks the focused path still exists in service.tree; if the note was deleted/renamed the map falls back to the hub seed, and ‹ Back skips history entries whose note is gone.
2. Vault-empty vs filtered-to-empty: an empty vault shows the map empty state with the spec's "showing 0 of 0 notes" caption; filters removing everything shows "No notes match these filters" with a Reset affordance.
3. Full ScoutTests target run before each commit — 429 tests in 79 suites, 0 failures (no -only-testing false greens).

Inline nits: no try on the non-throwing reparseAndWait(); canvas .id keyed on the sorted rawValue join of active types (not the set count); spec updated with the tap-divergence sentence, the global-degree "Hide orphans" note, and a neutral search example.

Verification

• xcodebuild build -scheme Scout -destination 'platform=macOS' — BUILD SUCCEEDED.
• Whole ScoutTests target after every task: 429 tests in 79 suites passed.
• ⚠️ The plan's manual /run GUI walkthrough (tap-to-re-root, filter toggles) could not be driven autonomously: this machine has no accessibility grant for UI scripting and the env-gated boot-tab harness was never merged, so the KB tab can't be reached programmatically. Please give the map a quick manual spin before merging.

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