0.3.4 — Cancel button, ETA and richer merge-report.json

New

• Cancel button on the merge progress screen. Clicking it interrupts the worker thread; the running merge aborts cleanly, the partial .part file and temporary extract dirs are removed, and the user lands back on Select Replay with nothing damaged.
• Estimated time remaining in the progress text — e.g. Streaming merge... tick 12345 / 50000 — ETA 1m 27s. Recomputed every 10 000 packets from the elapsed wall-clock time. No estimate is shown for the first few seconds (avoids "ETA 99h" jitter on a cold start).
• Richer merge-report.json: each merged replay now carries a mergedAt ISO-8601 timestamp, the producing multiviewVersion, and the absolute source path of every input replay. Useful for debugging "which file actually went into this merge".

Behaviour

• No change to merge output bytes for a given input set.
• Cancellation propagates via thread interrupt + a periodic Thread.interrupted() poll inside the streaming loop, so a cancel takes effect within roughly one packet batch (~10 ms in practice).
• The progress screen now shows two bottom buttons side-by-side: Cancel (active during the merge) and Back (active only after success / failure / cancel).

Compatibility

Same MC matrix as 0.3.3: 1.21.9 / 1.21.10 / 1.21.11 / 26.1 / 26.1.1 / 26.1.2. The new UI elements are present on both the classic and modern UI variants.