docs(readme): lead 'What it does' with the v3.0 four-lane retrieval stack

README.md

@@ -55,14 +55,16 @@ That's it. Your next prompt that mentions "push" already has the rule attached.
 
 ## What it does
 
-When you submit a prompt in Claude Code, aelfrice's `UserPromptSubmit` hook fires before the model sees your message. It runs a two-layer search:
+When you submit a prompt, aelfrice's `UserPromptSubmit` hook fires before the model sees your message. It runs four retrieval lanes in parallel and merges the result:
 
-```
-L0: locked beliefs   -> rules you marked permanent (always returned)
-L1: FTS5 keyword     -> SQLite full-text search, BM25-ranked
+```text
+L0: locked beliefs   -> rules you marked permanent (always returned, never trimmed)
+L1: FTS5 keyword     -> SQLite full-text search, BM25 + posterior-weighted rerank
+L2: graph walk       -> typed-edge BFS from L1 seeds (SUPPORTS, CONTRADICTS, SUPERSEDES, DERIVED_FROM, ...)
+L2.5: structural HRR -> Plate-FFT bind/probe against anchor text + structural markers
 ```
 
-(Since v3.0, two additional lanes run alongside L1 by default: an L2 graph walk over typed edges from L1 seeds, and an L2.5 structural HRR rerank against anchor text and structural markers. The two-layer phrasing above is the v1.0 baseline; the four-lane stack is the v3.0 default. See [ARCHITECTURE § Retrieval](docs/ARCHITECTURE.md#retrieval).)
+L0 always ships; L1, L2, and L2.5 are budget-trimmed against the merged candidate set in score-descending order, with locked beliefs winning every overflow. Default token budget is 2,400. The default ranking stack flipped to `stack-r1-r3` (entity expansion + per-store IDF clipping) at v3.0; bench evidence on the labeled query-strategy corpus measured **+0.2851 absolute NDCG@k (+94.8%)** versus the v1.4 raw-BM25 baseline at p99 latency 4.5 ms. Full lane wiring (composition, latency budgets, federation peer DBs) is in [ARCHITECTURE § Retrieval](docs/ARCHITECTURE.md#retrieval).
 
 The matching beliefs come back as an `<aelfrice-memory>` block prepended to your prompt. The agent reads it as part of the prompt — it doesn't have to remember to check a file.
 
@@ -157,8 +159,6 @@ Running in the background. No action required after `aelf setup`.
 - **Local-only.** SQLite at `<repo>/.git/aelfrice/memory.db`. No telemetry, no network calls, no accounts. Per-project isolation by construction. See [PRIVACY.md](docs/PRIVACY.md).
 - **Removable.** `aelf uninstall --archive backup.aenc` encrypts the DB to a file, then deletes it. Or `--purge` for a full wipe.
 
-Tradeoff: no fuzzy semantic recall. See [PHILOSOPHY.md](docs/PHILOSOPHY.md).
-
 ---
 
 ## Day-to-day surface