docs: refresh README against implementation and guard against drift

Bring README.md and RSYNC_COMPAT.md in line with the actual code/CLI and add
an automated guard so they cannot silently drift again.

Waves A-E:
- Correct stale compatibility claims: archive is `-rlptD` (owner/group are
  opt-in via identity flags, not implied), and symlinks, hard links, xattrs,
  ACLs and `--dirs` are implemented.
- Remove documented-but-nonexistent features: the six unread FASTSYNC_* env
  vars, and `--client-cn` (server-only) from the client table.
- Repair the corrupted "Implementation Details" section (broken list numbering
  and emphasis) and correct it against the source.
- Sync the client and server option tables with usage.c / server_cli.c, and
  document server-contacting `--dry-run` (protocol 2.21.0).
- Hygiene: `# FastSync` heading, real build commands, consistent binary names,
  runnable TLS examples, daemon module keys.

Also align the client `--help` / archive log wording and the RSYNC_COMPAT
archive rows with the opt-in ownership model, and add
tests/integration/test_readme_consistency.py (marked `ci`) asserting every
documented FASTSYNC_* var is read in src/ and every documented client/server
flag appears in the corresponding `--help`.

tests/test_client_cli.c

@@ -188,7 +188,7 @@ static void test_cli_help() {
 }
 
 /* Test that the -a short spelling applies --archive's config bundle, matching
- * rsync -rlptgoD semantics: links + metadata + devices + specials, and NOT
+ * rsync -rlptD semantics: links + metadata + devices + specials, and NOT
  * compression/multithreading.  (--archive itself is covered by
  * test_parse_args_archive; this guards the short alias.) */
 static void test_cli_archive_flags() {