docs: reflect real posterior-draw architecture in AGENTS/README/HANDOFF

- AGENTS.md: Replace §4 (API Endpoint Availability) with updated text; insert
  new §5 (Real posterior draws via duckdb-wasm) documenting the shift from
  synthetic normal-approximation draws to client-side fetches via duckdb-wasm
  against the public Hugging Face parquet dataset. Include actual payload size
  (~39MB uncompressed / ~8.86MB gzipped). Renumber subsequent items.
- README.md: Update Chart 5 description in "What It Does" to reflect real draws
  + fallback behavior. Update API table to clarify that /api/v1/draws is not
  called from app but informs the Hugging Face URL the app fetches directly.
- HANDOFF.md: Mark "Raw posterior draws" as done (2026-08-11) with reference
  to the design spec.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -24,7 +24,7 @@ Visitors select a U.S. state, search for a school district (with suggestions of
 2. **Rate by student group** (Chart 2) — bar chart, most recent year (observed vs. modeled), SVG
 3. **District vs. national** (Chart 3) — highest-rate group compared to the U.S. average, SVG
 4. **Model predictions vs. observed** (Chart 4) — four quadrants (one-year/three-year × baseline/covariate)
-5. **Predicted rates by student group** (Chart 5) — D3 density ridges showing posterior distributions with diamond markers for observed rates
+5. **Predicted rates by student group** (Chart 5) — density ridges built from each group's real 500-draw posterior (fetched client-side via duckdb-wasm from the public Hugging Face parquet dataset), with diamond markers for observed rates; falls back to an analytic approximation if the draws can't be fetched.
 6. **Exceedance probability** (Chart 6) — P(district > national) per student group
 
 ## Architecture
@@ -72,7 +72,7 @@ crdc-demo/
 | `/api/v1/models` | List available Bayesian model specs | Once (cached) |
 | `/api/v1/districts?q=&state=` | District name/geo lookup → LEAID | On keystroke |
 | `/api/v1/estimates/{leaid}?model=X&year=Y` | Estimates for one district/model/year/group | ~40 calls per district |
-| `/api/v1/draws?...` | Locate raw-posterior Parquet shard (bulk only, not used in browser) | Not called from app |
+| `/api/v1/draws?...` | Locate raw-posterior Parquet shard | Not called from app — the app fetches shards directly from Hugging Face via duckdb-wasm; see `src/hooks/useDrawDistribution.js` |
 | `/data/national_rates.json` | Static national rates fixture (committed) | Once per session |
 
 ## Deployment