V-L1-A2: rewrite README around concerns octad (blocked by V-L1-A1)

[hyperpolymath]

Status

Blocked by V-L1-A1. Open this only after V-L1-A1 resolves in favour of the concerns octad. If V-L1-A1 picks modalities, close this with a pointer to V-L1-A3 instead.

Context

If the concerns octad ({Data, Metadata, Provenance, Lineage, Constraints, AccessControl, Temporal, Simulation}) is canonical, the README still leads with a modalities table that the codebase no longer supports as the primary identity. Reframing is needed.

What to do

In README.adoc:

1. Replace the "VeriSimDB's Octad: Eight Modalities" table with a table for the eight concerns. Map each to: purpose, sidecar table, code module, tier (Tier 1 for Provenance/Temporal; Tier 1 for AccessControl/Lineage/Constraints if the sidecar approach holds; Tier 2 for Simulation if it needs branching storage).
2. Rework the "Eight categories of cross-modal drift" section. Either:
   • move it under a "Drift detection" subsection that frames the eight categories as symptoms observed by the Constraints dimension when Data, Metadata, and the active overlays disagree; or
   • shrink it to a single paragraph that defers to a future modality-overlay doc.
3. In the "Tier 2: Augmentation Layer" section, retain modalities (graph, vector, tensor, semantic, document, spatial) but reframe them as overlay representations rather than as "the octad". A user enables an overlay because they want similarity search; this is independent of which concerns they enable.
4. Update the "Architecture" ASCII diagram to label the sidecars by concern (provenance log, lineage graph, temporal versions, access policies, simulation branches) and the Tier 2 boxes by modality (graph, vector, …).
5. Update ROADMAP.adoc Phase 0 / Phase 1 to talk in concerns, not modalities.

Acceptance

• README.adoc table heading reads "Octad: Eight Concerns" (or similar)
• README.adoc no longer claims modalities are "the octad"
• Modality overlays appear only under Tier 2 framing
• ROADMAP.adoc is consistent with the chosen ontology