docs(readme): rewrite around concerns octad (V-L1-A2) (#99)

Closes #20. ADR-0004 made the concerns octad canonical; the README
still led with the modalities table. This rewrite:

* Replaces the "Eight Modalities" table with an "Eight Concerns"
  table mapped to (question, sidecar table, code module, tier) per
  the issue's acceptance criteria.
* Recasts the drift section as **symptoms of Constraints-violation
  across modalities** rather than a free-standing taxonomy. The
  eight categories are listed with their default thresholds and a
  pointer to ADR-0003 for the (input, distance, threshold) triples.
* Reframes Tier 2 modalities (graph, vector, tensor, semantic,
  document, spatial) as *overlay representations* — alternative
  shapes the same entity is projected into for a specific query
  workload, independent of which concerns are active. The Simulation
  concern is also Tier 2 per ADR-0006.
* Updates the architecture diagram so Tier 1 boxes are labelled by
  concern (provenance log, lineage DAG, temporal versions, access
  policies, constraint/drift index) and Tier 2 boxes by modality
  (graph index, vector HNSW, tensor ndarray, semantic CBOR, document
  Tantivy, spatial R-tree), with simulation branches called out
  separately per ADR-0006.
* Adds a "Building and Running" section that names the canonical
  build paths from ADR-0009: cargo for dev (MSRV 1.85), Containerfile
  for ops. The experimental Nix/Guix/devcontainer paths are
  acknowledged but flagged as such.
* Cross-references the shipping implementations: V-L1-E2 (#49)
  Temporal drift detector as the first ADR-0003 category, V-L2-P1
  (#50) `verisimiser gc` for `[retention]` enforcement, the
  provenance threat model (#25), and ADRs 0003/0004/0005/0006/0007
  for the binding decisions.

`ROADMAP.adoc` Phase 0 / Phase 1 also rewritten: each Tier 1 bullet
is now per-concern (Provenance / Lineage / Temporal / AccessControl /
Constraints), with the eight drift categories tracked as Constraints
sub-items so the one shipped today (Temporal) and the seven remaining
(Structural / Semantic / Statistical / Referential / Provenance /
Spatial / Embedding) are visible from one place.

No code change. Tests unaffected.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath