docs: book chapter for the unified PathQuery

[QuantumExplorer]

Context

PR 7 — the final PR of the unified PathQuery effort (stacked on #801 ← #800 ← #799 ← #798 ← #797 ← #795).

Adds The Unified PathQuery chapter to the mdBook, placed right after The Query System: read modes behind the Query encoding's version byte, the three read-mode shapes and their constructors, classify() as the one consensus-relevant shape decision, run_path_query / verify_path_query, both new V1 envelope layer kinds (axis descents with recomputed attestations, sum-budget windows with the verifier's fold replay), the GROVE_V4 gate table, and the relationship to the kept-first-class specialized surfaces. The Query System chapter gains a short pointer section.

mdbook build passes. Translations intentionally not touched (they lag by design; adding a chapter does not require translating it).

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📥 Commits

Reviewing files that changed from the base of the PR and between ba76763 and ed6eb81.

📒 Files selected for processing (3)

• docs/book/src/SUMMARY.md
• docs/book/src/query-system.md
• docs/book/src/unified-path-query.md

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[QuantumExplorer]

This is Claude. Rebased onto develop (the branch was stacked on the pre-squash #801 head) and brought the chapter up to date with everything that merged since it was written:

• TopK → RankedPage with the directional top-k/bottom-k reading.
• Tag 3 is AggregateOverValueRange { lo, hi, fold } — the chapter now carries the fold table (counts [3,1,5], band [2,10]: Population 2, Total 8), the entries-not-distinct-values note, and the fact that all four (axis, fold) cells are served since feat: dual-aggregate count secondary — count+Total served (closes #806) #809.
• All secondaries are dual-aggregate PCPS, documented as a security requirement, not a convenience (per the feat: dual-aggregate count secondary — count+Total served (closes #806) #809 audit: the single-aggregate node hashes share a preimage layout).
• Query-as-input, fold edition: the embedded descent payload carries no fold — dual-aggregate commitments mean one proof serves both questions, each verified against its own hash-bound number; the standalone envelopes echo and authenticate the fold instead. The chapter explains why the two wire paths differ here.
• read_mode shown as Option<Box<ReadMode>>, constructor list updated (new_axis_aggregate_over_value_range, bottom_k).

mdbook build passes locally; the sum-budget and classify sections needed no changes (match_limit was already correct).