feat(flatfile): warn on missing or mismatched log format-version

src/database_flatfile.h

@@ -24,8 +24,13 @@
 
 // --- Constants ---
 
-#define DIR_FLATLOG      "flatlog"
-#define FLATFILE_HEADER  "# profanity chat log — UTF-8, LF line endings\n# vim: set fileencoding=utf-8 fileformat=unix :\n"
+#define DIR_FLATLOG             "flatlog"
+#define FLATFILE_FORMAT_VERSION 1
+#define FF_VERSION_PREFIX       "# format-version: "
+#define FLATFILE_HEADER                                                   \
+    "# profanity chat log — UTF-8, LF line endings\n"                     \
+    "# format-version: 1\n"                                               \
+    "# vim: set fileencoding=utf-8 fileformat=unix :\n"
 #define FF_MAX_LINE_LEN  (10 * 1024 * 1024) /* 10 MB — reject lines longer than this */
 #define FF_MAX_LMC_DEPTH 100                /* max correction chain depth */
 
@@ -125,6 +130,11 @@ char* ff_unescape_meta_value(const char* val);
 // Skip UTF-8 BOM if present; returns 3 if skipped, 0 otherwise.
 int ff_skip_bom(FILE* fp);
 
+// Scan leading '#' comment lines for the format-version marker. Returns
+// the version found, 0 if no marker is present, or -1 on read error.
+// File position is rewound to the start of the comment block on entry.
+int ff_read_format_version(FILE* fp);
+
 char* ff_readline(FILE* fp, gboolean* truncated);
 void ff_write_line(FILE* fp, const char* timestamp, const char* type, const char* enc,
                    const char* stanza_id, const char* archive_id, const char* replace_id,