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>

migrations/V001__init.sql

@@ -0,0 +1,15 @@
+-- V001__init.sql — schema bootstrap.
+-- Per design §5.1 + §5.9. Only the meta + migrations tables land here;
+-- data tables (assets, documents, blocks, chunks, fts5, …) ship in later
+-- phase-specific migrations (P1-6 / P2-1 / P3-3).
+
+CREATE TABLE schema_meta (
+  key   TEXT PRIMARY KEY,
+  value TEXT NOT NULL
+);
+
+CREATE TABLE migrations (
+  id          INTEGER PRIMARY KEY,
+  applied_at  TEXT NOT NULL,
+  description TEXT NOT NULL
+);