docs: fix stale test/README claim — diffuser has runtime files

The README claimed "Placeholder structure (diffuser currently has no
runtime module files)" but `src/nodeClass.js`, `src/specificClass.js`
(284-line OTR/ΔP model), and `src/commands/` all exist. Updated to
describe the actual layout.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

test/README.md

@@ -1,3 +1,11 @@
 # diffuser Test Suite Layout
 
-Placeholder structure (diffuser currently has no runtime module files).
+`test/{basic,integration,edge}/` — standard EVOLV three-tier test
+structure. The diffuser runtime modules live at `src/nodeClass.js`,
+`src/specificClass.js`, and `src/commands/`; they implement the OTR /
+ΔP aeration model and the `data.airFlow` / `set.kla` topic handlers
+documented in `CONTRACT.md`.
+
+Helpers shared across tiers belong in `test/helpers/`.
+
+Run: `node --test test/basic test/integration test/edge`.