Derive absolute subtree-path for NL topological-lock enforcement (#13056 leaf)

Context

Leaf of #13056. The cross-family review of #13126 (LockRegistry) required, as a Post-Merge contract, that "the enforcement slice constructs subtreePath as the absolute root→node component-id path" — because LockRegistry.pathsOverlap is sound only for absolute paths (a relative path lets unrelated subtrees sharing a head id false-overlap). This leaf delivers that derivation as a pure, reusable function: the bridge between a live component id and the subtreePath that WriteGuard (#13134) / LockRegistry (#13125) consume.

Release classification: boardless — harness coordination line. Unblocked (off dev; does not import LockRegistry/WriteGuard).

Scope (this leaf — pure)

deriveSubtreePath(componentId, parentOf):

• walks the parent chain from componentId up to the root via the injected parentOf(id) → parentId|null lookup, returning the absolute root→node id array [rootId, …, componentId];
• stops at a falsy parent or Neo's top-level sentinel 'document.body' (the top component is the root; the body is not a component);
• cycle-guarded (a repeated id in the chain → fail-closed null, never an infinite loop);
• fail-closed on a malformed componentId (null).

parentOf is injected so the function is pure and unit-testable with a mock tree; the wiring supplies the live lookup (id => Neo.getComponent(id)?.parentId, built on Neo.component.Base#parentId / #up).

Out of Scope (named follow-up: the enforcement-wiring slice)

• The live parentOf lookup + intercepting InstanceService.setInstanceProperties (and the other write-class client services) to call WriteGuard.requestWrite and deny on conflict — the integration / whitebox-e2e slice.
• WriteGuard itself (#13134) and LockRegistry (#13125).

Contract Ledger

Decision Record impact

Satisfies the #13126 Post-Merge absolute-subtreePath contract. aligned-with ADR 0020 guardrail #1. No ADR amendment.

Acceptance Criteria

• deriveSubtreePath returns the absolute root→node id path via an injected parentOf, unit-tested (linear chain; root and document.body stops; mid-chain absent parent; cycle → null; malformed id → null).
• Pure + side-effect-free — no live-heap / Bridge import (the connect-on-init-singleton trap is avoided).
• Evidence: L1 pure-derivation unit coverage; the live wiring (real Neo.getComponent lookup at the write site) is the named e2e residual.

Related

Parent: #13056. Refs #13012, #13125 / PR #13126 (the absolute-path contract), #13134 / PR #13135 (WriteGuard consumer), src/ai/client/InstanceService.mjs:75.

Origin Session ID: 4c598c8f-d8a7-4288-9420-e825a45d310e Retrieval Hint: "derive absolute subtree path root to node component id parentOf cycle-guard NL topological lock enforcement subtreePath"