Say where the log is in the guides an operator actually reads

docs/deployment.md

@@ -276,6 +276,50 @@ If `CMakeLists.txt` or options changed, re-run the `cmake -B build ...` configur
 [section 2](#2-build-full-with-cameras--mqtt) first. To roll back, keep the previous binary
 (`fire_gimbal_control.prev`) and swap it back.
 
+## 10. Where the log is
+
+The program writes a persistent log alongside the self-test and diagnostics
+reports:
+
+```
+~/.local/share/fire_gimbal_control/logs/fgc_<YYYYMMDD-HHMMSS>.<NNN>.log
+```
+
+One file per run, rotated by size within a run (`.000`, `.001`, …), oldest pruned
+by count. Bounded at `[Logging] file_keep x file_max_mb`, 200 MB by default, at
+roughly 5 MB/day in steady state.
+
+This exists because a tower runs detached and, in TUI mode, every line otherwise
+goes to a 500-line on-screen ring buffer that dies with the tmux session. The
+file is **more verbose than the dashboard** — `info` on screen, `debug` on disk —
+so it holds detail the pane never showed.
+
+```bash
+ls -t ~/.local/share/fire_gimbal_control/logs/fgc_*.log | head
+tail -F "$(ls -t ~/.local/share/fire_gimbal_control/logs/fgc_*.log | head -1)"
+```
+
+Use `tail -F`, not `-f`: rotation replaces the file and `-f` would follow the old
+inode into a file nothing writes to any more.
+
+Each file opens with a header naming the build, tower, PID, config and hardware
+paths and the full command line — repeated after every rotation, so an archive
+found on its own still says where it came from. Lines carry a `YYYY-MM-DD` prefix
+the console ones do not.
+
+What to grep when a tower will not come up:
+
+```bash
+L=$(ls -t ~/.local/share/fire_gimbal_control/logs/fgc_*.log | head -1)
+grep -E "hardware:|NEED|firmware config|need config" "$L"   # declaration + handshake
+grep -E "\[WARN |\[ERROR" "$L"                              # everything it complained about
+head -8 "$L"                                                # which build, which config
+```
+
+> Wire traces (`[Logging] trace` / `--trace`) reach this file too. The serial
+> trace alone takes it from ~5 MB/day to ~41 MB/day, enough to age out the
+> startup record within days — turn it on to chase something, off afterwards.
+
 ## Troubleshooting
 
 | Symptom | Check |

docs/tower-installation.md

@@ -867,6 +867,24 @@ Check against the recorded figures: travel ~1,210,778 counts, repeatable to
 more than a few percent means the axis homed against something that was not the
 endstop.
 
+### Reading the log afterwards
+
+Everything below is recorded to
+`~/.local/share/fire_gimbal_control/logs/fgc_*.log`, not only to the dashboard —
+which matters here, because in TUI mode the pane holds 500 lines and dies with
+the tmux session. During this bring-up that cost us the handshake record four
+separate times before the file existed.
+
+```bash
+L=$(ls -t ~/.local/share/fire_gimbal_control/logs/fgc_*.log | head -1)
+grep -E "NEED|firmware config|need config" "$L"   # step 4: did the handshake complete
+grep -E "hardware:"                        "$L"   # presence reconciliation
+grep -E "speed:|motor current"             "$L"   # what was actually sent to the firmware
+```
+
+The file is more verbose than the dashboard was, so it holds lines the pane never
+showed. `head -8` gives the build, tower and config that produced it.
+
 ### If it goes wrong
 
 The board is recoverable over the network as long as USB CDC comes up: