Implement D79: deterministic structure skeleton + bottom-up summaries consumed by E1/E2

[fazpu]

Owner-approved 2026-07-27 ("this suggestion is really good and should be done - we need the system to scale"). Design: PR #164 — D79 in decisions.md + plan/designs/e0_files_design.md §4.1.

Scope:

1. Deterministic markdown-heading parser for the section skeleton; LLM fallback for structureless documents returns searchable heading strings, never char offsets. §4 output contract unchanged (document_sections on the block grid, pageindex.json sidecar).
2. Bottom-up summaries: flash-class summary seat (D70 port binding), one bounded call per leaf section, parents composed from child summaries; parallel; content-hash cached per section.
3. Consumption: summaries into the E1 context-prefix input and the E2 bundle as the D31 "section path + summary" element (target + ancestor summaries); added_context gains a summary tag with grounding-gate verification; extractor version bump. Closes the summary half of E2 context bundle diverges from the D31 design: section summary and entity hints are missing #163.
4. Provenance: split structurer_version into skeleton version + summary seat/version.

Sequencing: after #161 (loss ledger) merges, so the bundle change lands with measurement in place. Judge summary quality only where consumed (#150 canaries).

[fazpu]

Scope update from the PR #164 review cycle (revision 68ac4c0): implementation must follow the revised D79 — notably: summaries are orientation-only (no summary added-context kind, excluded from extraction_input_hash); role assignment is its own pass (deterministic title rules → bounded title-only classifier → body) with its own generation; placement rides the root reduction call; leaf summary calls shard at block grain under a hard token ceiling and parent calls include parent-direct blocks; structure/summary generations are immutable with a current pointer and legacy backfill. The E2 bundle change still bumps the extractor version (bundle text changes) even though no new added-context kind is introduced.

[fazpu]

Implementation starts now (owner-directed), in three mergeable waves, each through the pipeline Codex implements → Grok + Opus review → Claude final review:

Wave 1 — the structure stage (branch feat/d79-structure-stage): blockizer heading metadata (raw level + normalized title, blockizer_version bump); deterministic skeleton parser; the normative stat schema; the skeleton-check seat (REMEMBERSTACK_SKELETON_CHECK_*, single-closed-enum verdict, check_outcome incl. failure states, fail-open); the string-anchor LLM fallback structurer (replacing the one-shot route); the terminal check with synthetic-root degrade; route tags + D52 append-only check records; five-generation provenance with legacy backfill. Placement is null in this wave (P3 type-default per the design's degraded path) until wave 2's root summary call exists.

Wave 2 — summaries + placement: summary seat, bottom-up sharded summaries with content-hash caching, placement on the root reduction, sidecar/generation wiring.

Wave 3 — consumption: summaries into the E1 prefix input and the E2 bundle element (orientation-only, no added-context kind, excluded from extraction_input_hash), extractor version bump.

Design of record: plan/designs/e0_files_design.md §4.1 as of 69891b6 + D79 and amendments in decisions.md.