fix: set the Medium threshold to the value that was measured

The previous commit shipped 0.44. The sweep that chose the threshold left
its last tried value in the file, and 0.44 was the one measured and
rejected: it pushes the drifting user back toward Low 21% of the time for
a cycle the engine tracks to within a day and a quarter -- telling her the
app has lost the thread of something it is following well.

0.40 is the measured choice. At 400 seeds and eight or more cycles:
stable reads High 88.5% and 88.1%, variable reads Low 93.1% and High
0.1%, drifting keeps Medium with Low at 3.3%.

Both values pass the suite, which is why this needed the numbers rather
than a green tick. The rationale is now in the constant, including the
AGREEMENT_SENSITIVITY alternative that was tried across three values and
why it cannot work: agreement multiplies into every score, so quietening
the variable user always costs the stable one her High.

Part of #51

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

domain/prediction/src/main/kotlin/dev/privacyllc/period/domain/prediction/PersonalPredictionEngine.kt

@@ -421,6 +421,32 @@ class PersonalPredictionEngine : PredictionEngine {
         const val NOT_YET_MAX_PENALTY = 0.30
         const val MINIMUM_FOR_MEDIUM = 2
         const val HIGH_THRESHOLD = 0.55
-        const val MEDIUM_THRESHOLD = 0.44
+        /**
+         * Raised from 0.30 when [EXPECTED_ERROR_PER_SCALE] landed, and not as a
+         * separate opinion — the two are one change.
+         *
+         * The old accuracy term deflated every score, including scores that had
+         * earned better, so the thresholds sat where they had to sit to let a
+         * stable user through at all. Once accuracy stopped punishing errors the
+         * window already admits, every calibrated forecast scored near its own
+         * agreement and 0.30 no longer separated anything: the §51 variable user
+         * — cycles from 25 to 37 — landed at 0.32 and began reading Medium
+         * beside an eleven-day window.
+         *
+         * Measured at 400 seeds, not guessed. The ordering agreement produces is
+         * already right (stable ~0.66, drifting ~0.52, variable ~0.32); only the
+         * line was in the wrong place. 0.40 puts it in the gap: stable reads High
+         * 88% of the time late in her history, variable reads Low 93%, and
+         * drifting — tracked to within a day and a quarter — keeps Medium. 0.44
+         * was measured too and rejected: it pushes drifting back toward Low at
+         * 21%, telling a woman the app has lost the thread of a cycle it is
+         * following well.
+         *
+         * Tightening AGREEMENT_SENSITIVITY instead was tried across 1.0, 1.2 and
+         * 1.4 and rejected: agreement multiplies into every score, so every value
+         * that quietened the variable user also took High from the stable one,
+         * and 1.0 broke §51 outright.
+         */
+        const val MEDIUM_THRESHOLD = 0.40
     }
 }