feat: page-preserving TreeKnowledge build + agentic navigation

@keli-wen is already working on this.

Since Jul 10, 2026.

## Feature Summary

Add page-preserving tree construction and agentic navigation for supported financial long documents: papers, filings, transcripts, and reports.

This complements semantic search rather than replacing it:

- semantic search finds candidates across a collection;
- agentic navigation reasons through one candidate document's hierarchy.

The semantic baseline in #111 and its paper integration in #112 land first.

## Motivation

Long financial documents do not fit in one extraction pass, and flat chunks lose document structure and page provenance. A page-anchored `TreeKnowledge` provides:

1. structure-aware navigation through titles and summaries;
2. page-anchored evidence through canonical citations;
3. one shared tree contract across supported financial document types.

VectifyAI/PageIndex validates the approach, but implementation choice remains open. Reuse or adapt low-level PageIndex mechanics where useful while preserving QuantMind's own tree, time, provenance, and citation contracts.

## Ownership

- `preprocess/`: preserve page boundaries and deterministic outline signals.
- `knowledge/`: reuse `TreeKnowledge`, `TreeNode`, and `Citation`; do not add a second `PageIndexNode` tree model. Extend `Citation` with optional `end_page`: `page` is the 1-based inclusive start, an omitted end means one page, and a present end must satisfy `end_page >= page`.
- `flows/`: build/enrich canonical trees and summaries with the Agents SDK.
- `library/`: persist canonical trees and provide semantic candidates through #111; this issue adds no second store or semantic index.
- `mind/navigation/`: perform agentic traversal over titles and summaries.
- `flows/` or an agent application: optionally compose semantic shortlist and tree navigation after both primitives exist.

## API Direction

```python
from quantmind.flows import extract_paper_knowledge
from quantmind.mind.navigation import navigate_tree
from quantmind.preprocess import build_page_outline

outline = build_page_outline(pages)
tree = await extract_paper_knowledge(outline, cfg=cfg)
hits = await navigate_tree(tree, question)
```

Per-domain public extraction names must follow `docs/design/en/operations.md`; shared enrichment helpers remain private. Results identify canonical item/node IDs and page evidence; they do not expose a provider-specific response type.

## Acceptance Criteria

- [ ] Runs after the semantic baseline in #111 is available for comparison.
- [ ] Preserves 1-based inclusive page spans from supported parsed inputs and validates `page <= end_page` when an end is present.
- [ ] Covers citation page-span serialization round trips.
- [ ] Builds a valid canonical `TreeKnowledge` without a second tree schema.
- [ ] Keeps deterministic page/outline work free of LLM calls.
- [ ] Keeps summary enrichment in `flows/` and navigation in `mind/navigation/`.
- [ ] Depends on #71 for a `mind` scaffold and import-linter boundary. If that scaffold has not landed, this issue adds the minimal package and enforces `mind -> library -> knowledge` without persistence/index ownership in `mind`.
- [ ] Returns canonical node IDs plus page-anchored evidence.
- [ ] Reuses `LocalKnowledgeLibrary` for persistence or semantic shortlist when needed.
- [ ] Includes offline tests and one focused example.
- [ ] Compares navigation quality with the semantic baseline on a bounded financial-document fixture.
- [ ] `scripts/verify.sh` passes.

## Non-goals

- A general-purpose RAG or PageIndex framework
- A second `PageIndexNode` or generic `Document` schema
- A second persistence or semantic-index layer
- Knowledge Graph construction
- A provider registry or retriever hierarchy
- Answer synthesis in the navigation primitive

## Related Issues

- Depends on #111 for semantic retrieval and canonical library persistence.
- Follows #112 for the first paper extraction-to-search baseline.
- Depends on #71 for the `mind` cognitive boundary.

## Implementation Considerations

- Functions over class hierarchies; no generic retriever abstraction.
- Standard parsing without OCR is acceptable for V1.
- The implementation may reuse PageIndex code or concepts subject to license, dependency, and contract review; do not commit to reimplementing commodity mechanics before that review.

### Breaking Changes

- [ ] This feature introduces breaking changes.
- [x] This feature is backward compatible.