Implement sync_logic.py and ffmpeg_export.py (build order steps 2-3)

sync_logic.py: pure delay_ms computation with the documented sign
convention. Caught and fixed a sign error in my own test-fixture
ground-truth doc while writing the unit tests: the audio+350ms fixture
actually requires delay_ms=-350 (delay video), not +350, since the
audio is already the late one.

ffmpeg_export.py: ports ffdelay's two-branch command structure
(offset video input vs audio input depending on delay_ms sign) plus
its documented quirks (leading-zero float formatting, -ss/-t applied
to both inputs for clip mode, non-fatal stream-copy warnings surfaced
via stderr rather than swallowed).

Added an end-to-end integration test (scripts/test_export_roundtrip.py)
that independently re-detects event timing in the exported output via
ffmpeg scene-change/silence detection (not the same code path used to
build the export) and confirms both the positive- and negative-delay
branches bring drift down from ~350ms to under 1ms.

sync_logic.py

@@ -0,0 +1,45 @@
+"""Pure marker/offset math -- no UI, no ffmpeg, no mpv. Easily testable
+in isolation per the project's suggested build order.
+"""
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class SyncOffset:
+    delay_ms: float
+
+    @property
+    def target(self) -> str:
+        """Which track the export step needs to delay to fix sync."""
+        if self.delay_ms > 0:
+            return "audio"
+        if self.delay_ms < 0:
+            return "video"
+        return "none"
+
+    def describe(self) -> str:
+        if self.delay_ms == 0:
+            return "Offset: 0 ms (already in sync)"
+        sign = "+" if self.delay_ms > 0 else ""
+        action = "audio will be delayed" if self.delay_ms > 0 else "video will be delayed"
+        return f"Offset: {sign}{self.delay_ms:.0f} ms ({action})"
+
+
+def compute_offset(video_mark_sec: float, audio_mark_sec: float) -> SyncOffset:
+    """video_mark_sec / audio_mark_sec are timeline positions (seconds) in
+    the *currently loaded* file where the same real-world event (clap/flash/
+    beep) was observed on each track.
+
+    delay_ms = (video_mark - audio_mark) * 1000
+
+    Sign convention (matches PROJECT_BRIEF.md and ffmpeg_export.py):
+    - delay_ms > 0: the audio event appears *earlier* in the timeline than
+      the video event, i.e. audio is ahead -- it needs to be delayed
+      (shifted later) to line up with video.
+    - delay_ms < 0: the audio event appears *later* than the video event,
+      i.e. audio is already behind -- delaying it further would be wrong.
+      Instead the video gets delayed by the same amount, which catches
+      video up to where the (late) audio already is.
+    - delay_ms == 0: already in sync.
+    """
+    return SyncOffset(delay_ms=(video_mark_sec - audio_mark_sec) * 1000.0)