PROTO-0271G - TraversalOptic cursorized traversal

[flyingrobots]

Summary

Add TraversalOptic as cursorized multi-hop movement over NeighborhoodOptic expansion with explicit strategy, direction, filters, max depth, max nodes, max edges, optional goal, and completeness.

Parent Design

• PROTO-0271 - Holographic slicing and streaming checkpoint basis #626
• docs/design/0271-v18-holographic-slicing-checkpoint-basis/v18-holographic-slicing-checkpoint-basis.md

Required Proof

Tests must prove cursor frontier state, deterministic expansion, open-frontier reporting, goal-not-found-within-boundary distinction, and global-scan obstruction for unbounded requests.

Roadmap Slot

• Release lane: v18.0.0
• Feature area: materialization-query-index
• This issue is one implementation slice from PROTO-0271.

[flyingrobots]

Design doctrine update for this PROTO-0271 slice landed in PR #641, commit 8969ce73.

Relevant doc anchors:

• docs/design/0271-v18-holographic-slicing-checkpoint-basis/v18-holographic-slicing-checkpoint-basis.md now has an Echo-Derived WARP Doctrine section.
• docs/ARCHITECTURE.md now has a Graph-shaped readings section.

Implementation consequence for this issue: treat witnessed causal history as authority; graph-shaped values are observer-relative readings; Git/CAS byte hashes, retained payload hashes, commitment/proof refs, basis identity, and semantic reading identity must stay separate; missing witness/rights/budget/support evidence must surface as obstruction/residual/redaction/plurality/rehydration posture, not hidden full-materialization fallback.