Record that lowering the noise floor to 8s is wrong

The 12s floor cut real detection on shows with verified ads -- LeVar
Burton Reads fell from 19.0 to 10.5 minutes found against a 26.6 minute
gap -- so 8s looked like the fix. Measured on an isolated copy of the
live data it is not: Sleepy History at 8s finds 174 minutes of
"advertising" in a show that has 48 seconds of it, worse than the 4s
floor that produced Peter's original complaint.

MinRunSamples and MergeGapMs interact and cannot be tuned separately. A
low floor lets noise fragments survive and the 60s bridging then welds
them into enormous spans.

Had only LeVar been measured the floor would have been lowered.

Also re-pins .coverage-baseline, which the hook left modified after
d96c876 deleted covered readout code.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018AwnALZL6bD4VBtATDVv3B

1 files changed, 39 insertions, 0 deletions

diff --git a/docs/detection-findings.md b/docs/detection-findings.md
index 6f48b72..f3eb30f 100644
--- a/docs/detection-findings.md
+++ b/docs/detection-findings.md
@@ -799,3 +799,42 @@ breaks" on 111 of them, 74 minutes in total. Those are not advertisements. They
 own recurring furniture -- intro, break cue, outro -- which is worth skipping but is a different
 thing, and reporting it as inserted advertising is wrong. Every mirrored episode has a usable
 declared duration, so this classification is always available.
+
+## Lowering the noise floor to 8s: measured, and badly wrong
+
+The 12-second floor was set on 2026-10-15 after Peter reported "madness" — 37 breaks in one
+episode, 14 of them exactly 4.0s, the old floor. It worked: his episode went 37 breaks to 1, and
+every sub-12s fragment in the corpus disappeared.
+
+But it also cut real detection on shows with verified advertising. LeVar Burton Reads went from
+19.0 minutes found to 10.5, against the 26.6 the length gap implies. That looked like the floor
+being too aggressive, and an 8-second floor looked like the fix.
+
+**It is not.** Measured 2026-10-19 on an isolated copy of the live data, same episodes, same
+batch, production code with only `MinRunSamples` changed:
+
+| show | 12s floor (shipped) | 8s floor | length gap implies |
+|---|---|---|---|
+| LeVar Burton Reads | 10:30 | **19:43** | 27:35 |
+| Sleepy History | 14:11 | **174:44** | 0:48 |
+
+Sleepy History at 8 seconds finds **174 minutes** of "advertising" in a show that has 48 seconds
+of it — worse than the 48:03 the original 4-second floor produced, and three hours of a listener's
+programme proposed for deletion.
+
+**The two parameters interact, and that is the whole lesson.** `MergeGapMs` was widened to 60s in
+the same change that raised the floor. A high floor plus wide bridging is fine: few fragments
+survive, and bridging joins the ones that genuinely belong together. A low floor plus wide bridging
+is catastrophic: scattered noise fragments survive *and* get welded into enormous spans. Neither
+parameter can be tuned by looking at it alone, and the 8s number was chosen by post-hoc filtering
+of already-detected breaks, which cannot model a detector-level change — the same modelling mistake
+that produced the wrong "31% to 34%" prediction for the floor raise itself.
+
+**The floor stays at 12 seconds.** The cost is real and should be stated plainly: on shows with
+verified ad insertion the scan now finds roughly 38% of what the feed implies, down from about 69%
+at the old floor. That is the price of not proposing to skip three hours of a sleep podcast. Buying
+the detection back needs a mechanism that can tell a real short ad from a short coincidence —
+Peter's manual-skip logging idea is the obvious candidate, since it supplies labels neither the
+detector nor the feed can.
+
+Had only LeVar been measured, the floor would have been lowered.