refactor(flatfile): drop dead cursor_offset, guard inverted range

src/database_flatfile.c

@@ -54,7 +54,6 @@ ff_state_new(const char* filepath)
 {
     ff_contact_state_t* state = g_malloc0(sizeof(ff_contact_state_t));
     state->filepath = g_strdup(filepath);
-    state->cursor_offset = -1;
     state->archive_ids = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, NULL);
     state->stanza_senders = g_hash_table_new_full(g_str_hash, g_str_equal, g_free, g_free);
     return state;
@@ -294,7 +293,6 @@ ff_state_ensure_fresh(ff_contact_state_t* state)
     if (st.st_ino == state->stamp.inode && st.st_size > state->stamp.size)
         return _ff_state_extend(state);
 
-    state->cursor_offset = -1;
     return _ff_state_build(state);
 }
 
@@ -726,10 +724,6 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
     if (state->n_entries == 0)
         return DB_RESPONSE_EMPTY;
 
-    // Reset cursor for filtered (non-Page-Up) queries
-    if (start_time)
-        state->cursor_offset = -1;
-
     // Determine read byte-range using the sparse index
     off_t read_from = state->bom_len;
     off_t read_to = state->stamp.size;
@@ -789,6 +783,15 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
             read_from = backed;
     }
 
+    // Defensive: an empty or inverted range means there is nothing to read.
+    // Reachable when start_time bisects past end_time, or when an end_time
+    // upper-bound shrinks read_to below the BOM-adjusted read_from.
+    if (read_from >= read_to) {
+        log_debug("flatfile: empty range [%ld, %ld) for %s",
+                  (long)read_from, (long)read_to, contact_barejid);
+        return DB_RESPONSE_EMPTY;
+    }
+
     // Open file and read lines in [read_from, read_to)
     FILE* fp = fopen(state->filepath, "r");
     if (!fp)
@@ -861,10 +864,6 @@ _flatfile_get_previous_chat(const gchar* const contact_barejid, const gchar* sta
     if (!all_parsed)
         return DB_RESPONSE_EMPTY;
 
-    // Update cursor: byte offset of oldest parsed message in this batch
-    ff_parsed_line_t* oldest = all_parsed->data;
-    state->cursor_offset = oldest->file_offset;
-
     // Apply LMC corrections (if enabled) and build ProfMessage list
     GSList* pre_result = NULL;
     if (prefs_get_boolean(PREF_CORRECTION_ALLOW)) {

src/database_flatfile.h

@@ -88,7 +88,6 @@ typedef struct
     size_t cap_entries;
     size_t total_lines;
     ff_file_stamp_t stamp;
-    off_t cursor_offset;
     int bom_len;
     GHashTable* archive_ids;    // set: archive_id -> NULL (MAM dedup, O(1))
     GHashTable* stanza_senders; // map: stanza_id -> from_jid (LMC validation, O(1))