Rewrite Nano serial parser as line based, fix protocol bugs

- Commands run on '\n' instead of busy-waiting for a byte count, which
  fixes s/F/T waiting for the next command and P drawing 0xFF garbage
- Invalid bitmaps no longer leak their data as commands; bitmap data is
  streamed so full-screen bitmaps still fit
- Store flip time as uint16 with magic marker, validate 50-2000 us
- Overflow-safe millis() handling for the periodic refresh
- Run the full refresh only after D or when serial is idle, so incoming
  data is not lost while the redraw blocks
- Put the boot refresh measurement behind MEASURE_REFRESH
- Fix DEBUG hex output and use F() strings to save RAM
- Driver: direct mode now updates the old buffer instead of re-setting
  the new one

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/nano/README.md

@@ -8,18 +8,33 @@ logic.
 
 ## Behaviour
 
-- **Boot:** the Nano flips the whole display on and then off, then prints the time a full
-  redraw took.
+- **Boot:** the Nano loads the flip time from EEPROM and flips the whole display on and
+  then off.
 - **`display()`:** compares the new buffer with the last one that was shown and strobes
   only the dots that changed.
-- **Every 2 minutes** (`REFRESH_TIME`) it forces a full redraw to fix dots that got stuck.
-- **Flip pulse length** (µs) is set with the `T` command and stored in EEPROM, so it is
-  kept after a reboot.
+- **Every 2 minutes** (`REFRESH_TIME`) it does a full redraw to fix dots that got stuck.
+  A full redraw blocks for about half a second, which is long enough to overflow the
+  64-byte serial buffer. So it only runs right after a `D` (the end of a frame, when the
+  ESP is quiet) or when no serial data has arrived for 2 seconds.
+- **Flip pulse length** (µs, 50–2000) is set with the `T` command and stored in EEPROM,
+  so it is kept after a reboot. The default is 200 µs.
+
+### Debug options
+
+Uncomment these at the top of `main.cpp`:
+
+| Define | Effect |
+|--------|--------|
+| `DEBUG` | Print unknown command bytes on the serial port |
+| `MEASURE_REFRESH` | At boot, time a full redraw and print the duration in ms |
 
 ## Serial protocol
 
-Each command is one ASCII line. Coordinates are 2-digit **hex**. Color is `0` (black) or
-`1` (yellow). The ESP32 driver (`ESPSERIAL`) only uses `s`, `S`, `F`, `D` and `T`.
+Each command is one ASCII line that **ends with `\n`** (`\r` is ignored). The Nano
+collects a whole line before it runs the command. If a line is broken or unknown, it is
+skipped and the next line starts clean. Coordinates are 2-digit **hex**. Color is `0`
+(black) or `1` (yellow). The ESP32 driver (`ESPSERIAL`) only uses `s`, `S`, `F`, `D` and
+`T`.
 
 | Cmd | Format | Meaning |
 |-----|--------|---------|
@@ -27,10 +42,12 @@ Each command is one ASCII line. Coordinates are 2-digit **hex**. Color is `0` (b
 | `S` | `SXXYYc` | Set pixel and flip it right away (direct mode) |
 | `F` | `Fc` | Fill the whole buffer |
 | `D` | `D` | Push the buffer to the display (changed dots only) |
-| `P` | `PXXYYcsTEXT` | Print text at (XX,YY), color `c`, size `s`. `\n` in TEXT starts a new line. |
-| `B` | `BXXYYWWHHcHEX...` | Draw a bitmap. WW must be a multiple of 4. Pixel data is one hex digit per 4 pixels. |
+| `P` | `PXXYYcsTEXT` | Print text at (XX,YY), color `c`, size `s`. A literal `\n` (backslash, n) in TEXT starts a new line. The whole line can be at most 64 characters. |
+| `B` | `BXXYYWWHHcHEX...` | Draw a bitmap. WW must be a multiple of 4. Pixel data is one hex digit per 4 pixels, row by row. The data is drawn while it arrives, so it is not limited to 64 characters. |
 | `R` | `R` | Print the buffer as ASCII art on the serial port |
-| `T` | `Tnnn` | Set the flip pulse length in µs (decimal), stored in EEPROM |
+| `T` | `Tnnn` | Set the flip pulse length in µs (decimal, 50–2000), stored in EEPROM |
+
+Invalid commands get a reply that starts with `ERR:` on the Nano's TX pin.
 
 Example (what the ESP32 sends for one pixel and a refresh):