docs: update README banner — Alpha → Beta, remove fixed issues

- #249 (multi-node person counting) fixed by ADR-068 in v0.5.3
- #318 (training plateau) resolved
- Add #348 (n_persons overcount) as current known issue
- Add Cognitum Seed link for spatial resolution improvement

Co-Authored-By: claude-flow <ruv@ruv.net>

README.md

@@ -6,12 +6,11 @@
   </a>
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-> **Alpha Software** — This project is under active development. APIs, firmware behavior, and documentation may change. Known limitations:
-> - Multi-node person counting may show identical output regardless of the number of people (#249)
-> - Training pipeline on MM-Fi dataset may plateau at low PCK (#318) — hyperparameter tuning in progress
+> **Beta Software** — Under active development. APIs and firmware may change. Known limitations:
 > - No pre-trained model weights are provided; training from scratch is required
 > - ESP32-C3 and original ESP32 are not supported (single-core, insufficient for CSI DSP)
-> - Single ESP32 deployments have limited spatial resolution
+> - Single ESP32 deployments have limited spatial resolution — use 2+ nodes or add a [Cognitum Seed](https://cognitum.one) for best results
+> - Multi-person counting (n_persons) may overcount in single-occupancy scenarios ([#348](https://github.com/ruvnet/RuView/issues/348))
 >
 > Contributions and bug reports welcome at [Issues](https://github.com/ruvnet/RuView/issues).