[BUG] tx3g subtitle timestamps broken — integer overflow on GPAC path, off-by-one on FFmpeg path

## Description

tx3g (mov_text) subtitle extraction produces incorrect timestamps on both the GPAC and FFmpeg code paths.

## Reproduction

Create a test MP4 with tx3g subtitles:
Verify subtitles are correct:
Output (correct):
## Bug 1: GPAC path (master branch) — integer overflow
Output:
End timestamps are 0xFFFFFFFF — unsigned integer overflow. Start times are also wrong (both 00:00:00 instead of 00:00:01 and 00:00:04).

## Bug 2: FFmpeg/Rust path (feat/replace-gpac-with-ffmpeg branch) — timing offset

Output:
- Subtitle 1 start time is 00:00:00 instead of 00:00:01 (off by 1 second)
- Subtitle 2 end time is 00:00:09,965 instead of 00:00:06,000 (uses video end instead of subtitle end)

## Environment

- macOS arm64
- CCExtractor 0.96.5
- FFmpeg 8.0.1
- Test file: MP4 with H.264 video + tx3g subtitle track