Add continuity check: abort if clip audio doesn't actually overlap

Distinguishes a genuine audio mismatch at a clip boundary (evidence
the inputs aren't really a continuous sequence) from a boundary that's
simply too quiet to judge either way, using an RMS floor alongside the
existing cross-correlation confidence score. A confident mismatch now
aborts before encoding instead of silently splicing unrelated clips
together; --allow-discontinuous opts back into the old behavior.

README.md

@@ -21,7 +21,11 @@ This script:
    needed here).
 2. Cross-correlates the audio at each clip boundary to measure the *actual*
    overlap duration from the content itself (not just filename order or
-   whole-second creation timestamps, which aren't precise enough).
+   whole-second creation timestamps, which aren't precise enough). If a
+   boundary's audio simply doesn't match — a sign the inputs aren't actually
+   a continuous sequence (wrong order, a missing clip, unrelated files) — the
+   run aborts before encoding anything, rather than silently splicing
+   together clips that don't belong together.
 3. Trims the duplicated span off the start of each subsequent clip.
 4. Re-encodes the video across the joins (trimming mid-GOP HEVC can't be done
    with a plain stream copy) and concatenates everything into one file.
@@ -75,6 +79,22 @@ python .\concat_live_clips.py (2441..2445 | ForEach-Object { "IMG_$_.MOV" })
 | `--preset` | x265 preset (default: `medium`) |
 | `--metadata-from N` | Take container metadata from the Nth input clip, 0-indexed (default: `0`, the first clip) |
 | `--confidence-threshold N` | Minimum audio cross-correlation confidence required to trust an overlap detection before falling back to 0 (default: `0.9`) |
+| `--min-signal-rms N` | Below this audio RMS level (int16 scale), a boundary is considered too quiet to judge continuity, so a low-confidence match there won't trigger an abort (default: `25`) |
+| `--allow-discontinuous` | Proceed even if audio at a clip boundary doesn't match, instead of aborting |
 
 `--crf`, `--bitrate`, and `--lossless` are mutually exclusive; the default
 (no flag) auto-computes a bitrate target from the source clips.
+
+### Continuity check
+
+Before encoding, each clip boundary's audio is checked for a real match.
+Three outcomes:
+
+- **Confident match** (confidence ≥ `--confidence-threshold`) — the overlap
+  is measured and trimmed as normal.
+- **Too quiet to tell** (signal below `--min-signal-rms`) — treated as 0
+  overlap and the run proceeds, since there's no reliable signal either way.
+- **Confident mismatch** (enough signal, but it doesn't correlate) — treated
+  as evidence the clips aren't actually a continuous sequence, and the run
+  aborts with an error before any encoding happens. Pass
+  `--allow-discontinuous` to concatenate them anyway.