Closes #334.

Summary

• getNextOrPrevious gains an optional 5th isViable?: (nodeData) => boolean predicate. At every candidate leaf, the function synthesises NodeData for the candidate and calls isViable; on false, it recurses with the failed candidate as the new starting point. Sort-aware index derivation matches buildNodeData semantics so custom searchFilter / allowEdit callbacks see the same shape they get during render.
• useCommon composes isViableTarget from the precomputed filterState.visiblePaths Set (new useRawFilterState hook) and the existing allowEditFilter. Predicate is useCallback-stable across stable inputs. Threads it through getNextOrPreviousAtPath. KeyDisplay's Tab path (key-edit → value-edit) inherits the viability check for free.
• Redirect useLayoutEffect removed from ValueNodeWrapper. Tab no longer fires transient startEdit / cancelEdit observer pairs on dead-end nodes.

User-visible improvements

• Cleaner onEditEvent stream: a Tab past N non-viable nodes used to fire startEdit(badNode_1) → cancelEdit(badNode_1) → startEdit(badNode_2) → cancelEdit(badNode_2) → … → startEdit(goodNode). Now it's just startEdit(goodNode).
• No setState-during-useLayoutEffect pattern in every value node.
• O(1) Set-lookup visibility check instead of per-candidate filterNode calls (the visibility primitive built in Filtered collection counts + filter rewrite (#113) #351).

Behaviour changes worth flagging

• No previouslyEditedElement fallback. Today's redirect tries the previously-edited node before giving up; now when no viable Tab target exists, the editor cancels cleanly. Aligns with the "decision up front" model the issue argued for. recordPreviousEdit / previouslyEditedElement likely become dead state; queued for a follow-up cleanup PR once this is verified stable.
• Live search hiding the actively-edited node. The redirect's secondary role (cancel-when-active-becomes-invisible) is intentionally dropped. The node early-returns from render (its !isVisible guard), the input unmounts, the editing record stays in the store. Clearing the search later resumes the edit. No off-screen-commit footgun because there's no input element to commit through.

Test plan

• pnpm test keyboard — 6 new cases for the isViable predicate (skip single, skip run, return null when none viable, candidate-NodeData shape, descend-then-skip, no-predicate backward-compat). Total 21 passing.
• pnpm test JsonEditor — added 2 cases: Tab-past-non-editable fires NO transient observer events, and live search hiding the open node unmounts cleanly without error. Existing 'Tab and Shift+Tab skip filtered-out nodes' (line 1864) still passes — same user-visible behaviour via different mechanism. Total 107 passing.
• pnpm test — 456 tests across 18 suites passing.
• pnpm lint && pnpm compile && pnpm build — clean.
• Manual: Tab from a node where the next is filtered out → lands on the next visible. Tab past a non-editable node (allowEdit restriction) → skipped, no input flicker. Type into search while a node is being edited → input disappears, no errors. Clear search → input reappears, edit resumes. editorRef.startEdit(path, { force: true }) on a non-editable visible node → edit opens.
• Manual observer-event smoke: open the event-signals demo, Tab through a tree with mixed editable/non-editable nodes; the event stream should be clean.

Follow-up (separate PR)

After this lands and is verified stable, remove the now-unused previouslyEditedElement / recordPreviousEdit plumbing from EditingProvider and audit tabDirection for the same.

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