M3 step 5: Sweep, the band's signal strength as bars and a waterfall

Tab in the LoRa Scanner sweeps 863-870 MHz in 100 kHz steps (the
strongest of three RSSI readings at each, at 125 kHz), shown as bars with
peak hold over a waterfall, the Sniffer's frequency marked (Q98). The
Sweep pauses the Sniffer and keeps its packets; Tab resumes it (Q99).
Status Bar: SW. The floor, top and peaks are host-tested; `lora sweep
on|off|dump` prints them on the console.

At the desk: about 607 ms a pass, a flat floor at -100 to -102 dBm
(15 dB above the chip's own) and a steady carrier at 863.2 MHz.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

docs/milestones/M3.md

@@ -1,6 +1,6 @@
 # M3 — Radio bring-up: the LoRa Scanner
 
-**Status:** in progress (branch `m3`): steps 1 to 4 done.
+**Status:** in progress (branch `m3`): steps 1 to 5 done; the listening hour (step 6) is left.
 
 **Goal:** the LoRa radio on the Cap works, receive only: a Radio Service owns it and shares the SPI bus with the SD card safely, and a LoRa Scanner App shows what's on the air, either packets (Sniffer) or energy across the band (Sweep). Nothing in M3 can transmit. The mesh comes on top of this in M4 (receive) and M5 (transmit).
 
@@ -19,6 +19,7 @@
 - **Step 3:** the DIO1 interrupt works (a 100 ms receive timeout wakes the task after 105 ms). While listening: four 1.7 MB uploads and Gemini pages to the card, no radio or card errors (the card refuses a write about once in five uploads with the radio asleep too; `put` now catches it). The ring takes 9.8 KB while listening; the radio task's stack peaks at 2.0 KB.
 - **No packets yet, and a loud desk.** Twenty minutes on LongFast and on LoRaWAN's three uplink frequencies (SF7, SF9, SF12): no packet and no header, valid or not. The noise floor reads -83 to -94 dBm, against -112 dBm with the antenna switched off: 20 to 30 dB lost to something nearby, not the screen and not the GNSS receiver. Preamble detections are false alarms at this noise level (more on an empty frequency, 869.0 MHz, than on LongFast).
 - **Step 4:** a Capture made on the device reads back in TShark field for field (time, frequency, SF, RSSI, SNR, payload). A Capture keeps the radio listening with the App closed; stopping it puts the radio back to sleep.
+- **Step 5:** a pass across 863–870 MHz (71 steps, the strongest of three RSSI readings at each, measured at 125 kHz) takes about 607 ms. At the desk the band is **flat at -100 to -102 dBm**, about 15 dB above the chip's own floor at 125 kHz, with a steady carrier at 863.2 MHz (-89 dBm at its strongest) and fainter lines elsewhere: broadband noise from nearby electronics rather than a transmitter. Sweep's waterfall takes 2.6 KB while shown; 91.9 KB free during a Sweep. The radio task's stack peaks at 1.9 KB.
 - **Cost:** RadioLib 7.8.1 and the probe add 23.6 KB of flash and 656 bytes of static RAM to the release firmware (1,679,843 bytes of 3,342,336).
 
 ## Decisions (design round 2026-10-05)