docs: the three new features are described where each of their readers looks

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/claude/invariants-measurements.md

@@ -108,3 +108,21 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   `enters`, same `count[]`, identical `first_enter`/`last_enter` to 0.0 -- which is the assertion
   that the fast path is a fast path and not an approximation. The app's reading is
   `[measures-probe]` against `--noise 0.1`.
+- **A statistic's observation is one *recompute*, not one sample.** The pane already refreshes on
+  a cadence (at most one pass per 100 ms while the window scrolls, exactly once when it
+  settles), and the accumulator is fed from that pass's result — so `n` counts windows measured.
+  Feeding it per sample would make `n` a sample count and the mean a mean of *samples*, which is
+  not what any of these measurements measure: a frequency over a window is not a property of a
+  sample. The reading that names this defect is `[stats-probe]`'s `n` in the thousands rather
+  than the tens.
+- **The accumulator is Welford's, and the reason is the same one the time ring is `f64` for.** A
+  measurement's values are routinely large with a small spread — an absolute stream time is the
+  obvious case — and `Σx²  - (Σx)²/n` cancels to noise there. The standalone check is that the
+  spread of `{2,4,4,4,5,5,7,9}` survives a `+1e9` offset unchanged.
+- **What resets the statistics is "what is being measured changed", not "what is in view
+  changed".** An explicit Reset, an add/edit/delete (all three funnel through
+  `rebuild_measures_pane()`), a CONFIG, and a change of display window each change the
+  *question*, so the history is cleared. A pause, a resume, a trigger latch, a pan and a zoom do
+  not: they change which samples answer it, which is precisely what averaging over many windows
+  is for. The pane-wide toggle does not reset either — it only shows and hides labels that are
+  always created and always fed, so reading the plot for a moment does not cost the history.