feat(verisimdb): implement Gleam client JSON codecs — replace all stubs

Replace all scaffold stub decoders across the Gleam client SDK with
real JSON encoding/decoding using gleam/json and gleam/dynamic/decode.

The new codec.gleam module centralises serialization for all VeriSimDB
types: Octad (all 8 modalities), DriftScore, DriftStatusReport,
ProvenanceEvent, ProvenanceChain, VqlResult, VqlExplanation,
FederationPeer, FederatedQueryResult, SearchResult, and
PaginatedResponse. The OctadInput encoder now serializes all modality
data fields (graph, vector, tensor, document, spatial) rather than
only the modality name list.

Updated modules: octad.gleam, search.gleam, drift.gleam,
provenance.gleam, vql.gleam, federation.gleam.

Also documents in README.adoc that Burble uses the Elixir client,
not the Gleam client.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

verisimdb/README.adoc

@@ -443,6 +443,12 @@ verisimdb/
 └── .machine_readable/         # STATE.scm, META.scm, ECOSYSTEM.scm
 ----
 
+=== Client SDK Notes
+
+VeriSimDB ships six client SDKs in `connectors/clients/` (Rust, V, Elixir, ReScript, Julia, Gleam). Consumer projects should copy the relevant SDK into their own codebase rather than depending on this repo directly.
+
+**Burble** uses the **Elixir client** (`connectors/clients/elixir/`), which runs natively alongside Burble's Elixir control plane. The Gleam client is available for projects targeting the BEAM via Gleam, but Burble does not use it.
+
 == Use Cases
 
 === Data Quality at Scale