README: bring up to date

Old README was mid-rewrite and was missing two-thirds of what the
repo actually ships. Refreshed end-to-end:

- Replace stale 13-row RPC table (only listed the original PHY
  hacks) with an honest accounting: 46 request RPCs + 8 event
  channels covering Wi-Fi raw TX/RX, OCB, FTM, CSI, ESP-NOW, and
  802.15.4. Refer readers to host/include/esp_hosted_open.h for
  the canonical list.
- Front-load the cross-stack matrix (Wi-Fi a/g/n/ax + 802.11p OCB +
  FTM + CSI + ESP-NOW + 802.15.4 + BLE/HCI). BLE explicitly noted
  as upstream-already.
- Drop the "examples/p4_set_channel" reference — that directory does
  not exist. Replace with a runnable code example inline showing
  channel-set + ESP-NOW + FTM + promiscuous capture.
- Update "Still missing" — CSI, FTM, raw 802.11 TX, monitor-mode
  forwarding are now wrapped. Only BT-radio shockburst sequencer
  and a couple of housekeeping items remain.
- Refresh architecture diagram: slave overlay is now three files
  (phy_rpc_handlers / phy_rpc_extras / phy_rpc_wireless), and the
  block lists 2.4 GHz Wi-Fi, 5 GHz Wi-Fi, BLE, and 802.15.4 paths
  rather than just the 5 GHz radio.
- Mention the runtime capability bitmap up front — it's the key
  primitive that lets one host binary work across C5 / C6 / H2 /
  H4 slaves.
- Slave size figure updated from 27% free to 26% (it grew with the
  new wireless RPCs).
- Tighten intro paragraph; drop the "sub-GHz hack" line (no
  Espressif chip with a Wi-Fi blob has sub-GHz capability).

Author: DatanoiseTV