p0-1: wire-schema stubs, doc/spec stubs, V001 migration, fixtures

- docs/wire-schema/v1/ ships 7 schema stubs (citation, search_hit,
  answer, ingest_report, doc_summary, chunk_inspection, doctor) that
  pin schema_version + required fields per design §2. Full property
  validation lands in later phases.
- docs/spec/ ships 7 markdown stubs each linking to the canonical
  frozen design (domain-model, ids, canonical-document, chunk-policy,
  citation-policy, module-boundaries, ai-generation-guidelines).
- migrations/V001__init.sql contains only schema_meta + migrations
  tables per design §5.1; data tables ship in P1-6/P2-1/P3-3.
- fixtures/ has the 11 subdirectories every downstream task references
  (markdown, source-fs, search/{lexical,hybrid}, embed, vector, rag,
  eval, image, pdf, audio). Empty subdirs use .gitkeep so they track.
  fixtures/markdown/ ships the 3 phase-0 fixtures: simple-note.md,
  nested-headings.md, code-and-table.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/spec/ai-generation-guidelines.md

@@ -0,0 +1,12 @@
+# AI generation guidelines
+
+When implementing tasks against this codebase:
+
+- Treat the frozen design doc as the single source of truth. Do not invent
+  new fields, traits, or enum variants.
+- Prefer editing existing files to creating new ones; reuse types from
+  `kb-core` instead of duplicating shapes.
+- For each task, follow the task spec under `tasks/p<N>/p<N>-<i>.md`.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §11 + §12.

docs/spec/canonical-document.md

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+# CanonicalDocument
+
+Medium-agnostic representation of a document with `Block`s, `SourceSpan`s,
+and provenance.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §3.4 + §3.7a.

docs/spec/chunk-policy.md

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+# Chunk policy
+
+`ChunkPolicy` carries `target_tokens`, `overlap_tokens`,
+`respect_markdown_headings`, and `chunker_version`. Chunkers expose a
+`policy_hash` so chunk IDs include the policy.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §3.5 + §7.1 + §7.2.

docs/spec/citation-policy.md

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+# Citation policy
+
+Citations use W3C Media Fragments URIs to locate evidence inside a
+document. Five variants: `Line`, `Page`, `Region`, `Caption`, `Time`.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §3.5 + §0 Q3.

docs/spec/domain-model.md

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+# Domain model
+
+The domain types live in `kb-core` and mirror the frozen design exactly.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §3.

docs/spec/ids.md

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+# ID recipe
+
+All `kb-*` IDs are 32 hex chars: the first 32 of `blake3(canonical_json(tuple))`.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §4.

docs/spec/module-boundaries.md

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+# Module boundaries
+
+`kb-core` is leaf — every other crate depends on it. Parsers depend on
+`kb-parse-types` (not on `kb-normalize`); `kb-normalize` depends on
+`kb-parse-types` (not on parsers). UI crates depend only on `kb-app`.
+
+Canonical source:
+[docs/superpowers/specs/2026-04-27-kb-final-form-design.md](../superpowers/specs/2026-04-27-kb-final-form-design.md), §8.