Fix eta calculation

[8nt0n]

Fixes #85

Description

This PR resolves multiple accuracy, stability, and edge-case issues in the ETA calculation logic (moon_engine.py and app.js):

• ETA Clamp Fix: Updated raw_eta >= 7200 to return None (instead of the 7200 sentinel), allowing app.js to properly render --.
• Direct Remaining-Bytes Calculation: Replaced early-run average-file-size skew by calculating remaining bytes using known file_bytes for in-flight files and applying average estimates only to pending files without known sizes.
• Jitter Reduction: Decoupled the live UI speed display (3-second window) from the ETA calculation speed (10-second smoothed window).
• Terminal State Filter: Excluded non-active states (ok, fail, aborted, stopped) from _tracked so failed or aborted partial downloads no longer contribute ghost remaining bytes to the ETA.
• Code Cleanup & Tests: Cleaned up dead code (t_start), updated window comments, and added unit tests in test_snapshot_speed.py covering clamp behavior and terminal-state handling.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would change existing behavior)
• Documentation update
• Refactor / code cleanup
• Other:

Checklist

• I have tested my changes locally
• If this affects shared logic (extraction, download engine), I also applied the equivalent change to moon_cli.py
• I have kept the single-file architecture (no package split)
• I have not added new dependencies without justification in the PR description