chore: low-priority cleanups (logging, imports, deps, atempo, test hygiene)

[rsmxingu]

Bundles the low-priority review items into one PR. No user-facing behavior changes except the atempo edge-case fix.

Changes

Logging — one mechanism instead of two

_setup_logging() did a manual os.remove / truncate(0) dance and used a RotatingFileHandler. Replaced with RotatingFileHandler(mode="w"): each session still starts with a fresh log (the behavior documented in the README) and the rotating handler still caps in-session growth.

ui.py — hoist imports

import sys / import ctypes moved from inside __init__ to the module top.

processor.py — atempo chaining for extreme speeds

A single ffmpeg atempo only accepts factors in [0.5, 2.0]. Added _atempo_chain() which decomposes out-of-range speeds into a valid chain (e.g. 2.5 -> "atempo=2.0000,atempo=1.2500"). The common 1.2x path is byte-for-byte unchanged. This fixes a latent failure when auto-editor marks a clip faster than 2x. Applied to both the cut and no-cut audio paths.

requirements

• Added conservative upper bounds; pinned auto-editor<30 because the frozen-bundle integration relies on v29 internals (.v3 timeline, --no-seek, download_binary hook).
• Added requirements-dev.txt (pytest).

Test hygiene

• Renamed test_pipeline.py → integration_pipeline.py. It is a manual, ffmpeg-driven smoke script, not a unit test — the test_*.py name caused pytest to collect and run the whole pipeline on a normal test run.
• Added TestAtempoChain (4 cases).
• Fixed a stale progress-math comment in test_run_success_two_pass.

Deferred

Silence-threshold input validation was intentionally left out: it depends on messagebox, which is fixed in a separate PR ("import tkinter.messagebox explicitly"). Adding it here would couple the two.

Testing

• python3 -m py_compile processor.py ui.py tests/test_processor.py integration_pipeline.py passes.
• python3 -m unittest tests.test_processor → 12 passed.
• tests/test_ui.py requires tkinter (unavailable in my sandbox); please run python3 -m unittest tests.test_ui to confirm the ui.py import hoist there.

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