EPIC: Semantic and evidence substrate

Programme: #1

Own source-backed entities, observations, claims/evidence, retrieval, freshness/dependency invalidation and goal compilation. This layer keeps normalized data useful without erasing provider/account/source semantics.

## Child issues
- [ ] #41 DATA-01 — Entities, observations and evidence
- [ ] #42 DATA-02 — Hybrid retrieval with source-scoped authorization
- [ ] #43 DATA-03 — Dependency graph, invalidation and deletion
- [ ] #44 DATA-04 — Goal compiler and coordinated plans

## Cross-cutting gates
Source/user/inferred state stays distinct, access checks precede retrieval, derivatives invalidate correctly, and important displayed facts remain traceable to evidence.

Start by reading child issues #41–#44, which define entities and evidence, source-scoped retrieval, dependency invalidation, and goal compilation. Use the listed cross-cutting gates as completion criteria: preserve source/user/inferred distinctions, authorize before retrieval, invalidate derivatives correctly, and keep important displayed facts traceable to evidence.

Written by the indexing model from the issue text.