fix: Restore teletext auto-detect mode for single-page extraction

The page update logic at line 1029-1035 was incorrectly updating
tlt_config.page for all accepted pages, even in single-page auto-detect
mode. This caused the auto-detect logic at line 979 to be bypassed
because the first packet (even with an invalid page number like 0xFF)
would set tlt_config.page, preventing proper auto-detection.

The fix restricts the page update to multi-page mode only. In single-page
mode, tlt_config.page is set exclusively by:
1. User specification (--tpage option)
2. Auto-detect logic (first valid subtitle page found)

This fixes regression in SP Test 76 which uses sample
8c1615c1a84d4b9b34134bde8085214bb93305407e935edcdfd4c2fc522c215f.mpg
with --autoprogram --out=ttxt --latin1.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: cfsmp3

src/lib_ccx/telxcc.c

@@ -1023,9 +1023,10 @@ void process_telx_packet(struct TeletextCtx *ctx, data_unit_t data_unit_id, tele
 		if (!accept_this_page && !(de_ctr && flag_subtitle && ctx->receiving_data == YES))
 			return;
 
-		// Update tlt_config.page to track the current page being received
-		// This is needed so process_page knows which page to tag the output with
-		if (accept_this_page && page_number != tlt_config.page)
+		// Update tlt_config.page to track the current page being received (multi-page mode only)
+		// In single-page mode, tlt_config.page is set by auto-detect logic or user specification
+		// This prevents overwriting auto-detect selection with an arbitrary page number
+		if (is_multi_page_mode() && accept_this_page && page_number != tlt_config.page)
 		{
 			tlt_config.page = page_number;
 		}