README: update CLI reference and add diff mode section

- CLI table now shows which credentials file key each flag overrides
- Added --diff, --dry, --log-dir, -i/--interval, --host, --port
- New "Diff mode" subsection under Push server protocol

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

README.md

@@ -85,16 +85,22 @@ All connection and collection settings live in a single JSON file:
 
 ### CLI reference
 
-Only a handful of flags remain; all vehicle/server settings are in the credentials file.
+CLI flags override the corresponding credentials file value when given.
 
-| Flag | Description |
+| Flag | Overrides | Description |
-|------|-------------|
+|------|-----------|-------------|
-| `-c FILE` | Credentials / settings file (required) |
+| `-c FILE` | — | Credentials / settings file |
-| `-u / -p` | Override username / password (also `WC_USER` / `WC_PASS`) |
+| `-u / -p` | `credentials.username/password` | Username / password (also `WC_USER` / `WC_PASS`) |
-| `-d DOMAINS` | Override domains (comma-separated or `all`) |
+| `-d DOMAINS` | `domains` | Domains to collect (comma-separated or `all`) |
-| `--max-records N` | Keep only the last N records per file |
+| `-i SECONDS` | `interval_s` | Collection interval in seconds |
-| `-v` | Verbose / debug logging |
+| `--host HOST` | `host` | Push server bind address |
-| `--list-domains` | Print available domains and exit |
+| `--port PORT` | `port` | Push server TCP port |
+| `--log-dir DIR` | `log_dir` | Directory for rotating log files |
+| `--max-records N` | — | Keep only the last N records per file |
+| `--diff` | — | Send diffs instead of full snapshots (must match client setting) |
+| `--dry` | — | Skip WeConnect; run push server only (for testing) |
+| `-v` | — | Verbose / debug logging |
+| `--list-domains` | — | Print available domains and exit |
 
 **Available domains:** `charging`, `climatisation`, `measurements`,
 `readiness`, `parking`, `access`
@@ -138,6 +144,16 @@ The server listens on the `host`:`port` from the credentials file (default `0.0.
 
 All messages are **newline-delimited JSON** (one record per line, `\n` terminated).
 
+### Diff mode (`--diff`)
+
+When enabled on the server, the history burst is compressed: the first
+record is sent in full, subsequent records as `jay_diff_full` diffs
+(`{"_diff": true, "update_add": {…}, "delete": {…}}`).  Live snapshots
+are also diffed against the previous broadcast.  Enable `--diff` on the
+client side (`client.py`) or check **Diff mode** in the GUI Connector
+tab to reconstruct full snapshots from the stream.  Both sides must
+agree — a mismatch produces garbled output.
+
 ## GUI client (`gui_client.py`)
 
 ```bash