docs: restructure architecture docs to flowgraph pattern

- Create decisions/ directory with 32 numbered ADRs (ADR-001 through ADR-032)
  extracted from inline DD/SD/ED/SE decision sections
- Create open-questions.md with 16 OQs organized by theme, cross-referenced
  to ADRs, with status tracking (resolved/open)
- Create README.md as architecture index with doc table, ADR table, and
  lifecycle status definitions (draft/reviewed/stable/deprecated)
- Replace inline decision sections in all spec docs with ADR reference tables
- Replace inline open questions with OQ references to centralized tracker
- Update frontmatter: metagraph-module.md, overview.md, sqlite-host.md → reviewed;
  schema-evolution.md and encrypted-data.md remain draft
- DD1-DD10 → ADR-009 through ADR-018
- D1-D8 → ADR-001 through ADR-008
- SD1-SD5 → ADR-019 through ADR-023 (SD5 folded into ADR-006/008)
- ED1-ED5 → ADR-023 through ADR-027
- SE1-SE5 → ADR-028 through ADR-032

docs/architecture/decisions/015-edge-constraints-as-named-entries.md

@@ -0,0 +1,26 @@
+# ADR-015: Edge type constraints as named Module entries
+
+## Status
+
+Accepted
+
+## Context
+
+Edge type constraints (`allowedSourceTypes`/`allowedTargetTypes`) could be stored only as DB columns (JSON text arrays) or as first-class parts of the schema with validation and serialization.
+
+## Decision
+
+Edge type constraints are named Module entries (e.g., `TriggeredEdgeConstraints`), not just DB columns. This gives them schema validation (`Value.Check`) and serialization (JSON Schema with `$defs`). The repository layer projects these entries to the existing `edge_types.allowedSourceTypes`/`allowedTargetTypes` columns. The DB schema doesn't change — Module entries are the source of truth, DB columns are the persistence projection.
+
+Empty constraint arrays `[]` mean "no restriction" (any node type valid). Omitting the `*EdgeConstraints` entry means the same. An explicit entry with empty arrays is invalid.
+
+## Consequences
+
+- Constraints are validatable at the schema level, not just at the DB layer
+- `moduleToDbSchema()` extracts constraint entries to DB columns
+- Constraint data survives serialization/deserialization cycles via JSON Schema `$defs`
+- DB columns remain `allowedSourceTypes text` and `allowedTargetTypes text` with JSON arrays
+
+## References
+
+- [metagraph-module.md](../metagraph-module.md)