docs: check that the docs' headline figures match their committed data

Nothing had ever verified that a number written in prose matches the
reference data file committed beside it. The figure is written once from
a run; the prose is edited around it afterwards and the data file is
regenerated independently, so drift is silent.

All 19 headline figures across four censuses -- the eff-bit census, the
plateau census, the top-level rotation census and the eff-bit alpha test
-- currently agree with their data files.

The checker had to be numeric, and the first attempt is the reason it is
a script rather than a grep: comparing strings reported almost every
figure as a mismatch, because the data files write 14709 where the docs
write "14 709" with a thin space, and the docs round 33.66 to 33.7. A
consistency check that fails on formatting trains you to ignore it, so
the tolerance is explicit: exact against the data, within 0.05 against
the doc to allow rounding.

Also ran the full disc-gated workspace suite (build-reborn test, which
wires SYLPHEED_DISC -- without it the disc tests self-skip and green
means almost nothing), covering this session's three decoder changes:
rotation_deg on Keyframe, the scale-0 fix in blit/fill_quad, and the
flags field on T8adImage. 122 passed / 0 failed across the four suites
that had completed; the long disc-gated integration tests
(records_roundtrip_disc, first_header_word_is_record0_hash) were still
running and are not counted here.

docs/re/METHOD.md

@@ -712,3 +712,12 @@ agent's loop prompt, i.e. nowhere durable. See [`README.md`](README.md) for the
   call, but only if the negative space is written down — additive blend, name,
   lifetime, premultiplied alpha — so the next attempt starts where this one ended
   rather than at the beginning.
+* **Nothing was checking that a doc's figures match its committed data.** A
+  number is written once from a run and then lives in prose that gets edited
+  around it; the data file beside it is regenerated independently. All 19
+  headline figures across four censuses do currently agree
+  (`tools/re-capture/doc_figure_check.py`), which is worth knowing rather than
+  assuming — but the checker had to be written **numerically**, because the first
+  version grepped for the doc's formatting (`14 709` with a thin space, `33.7`
+  rounded from `33.66`) and reported almost every figure as a mismatch. A
+  consistency check that fails on formatting will train you to ignore it.

tools/re-capture/doc_figure_check.py

@@ -0,0 +1,48 @@
+#!/usr/bin/env python3
+"""Do the headline figures in a doc match its committed reference data?
+
+Numbers drift between a run and the prose written about it, and nothing was
+checking. This compares the two NUMERICALLY, which matters: a naive string
+grep reports every figure as a mismatch, because the data files write `14709`
+and the docs write `14 709` (thin space) and round `33.66` to `33.7`. That
+false-positive run is why this is a script and not a grep.
+
+    doc_figure_check.py            # runs the built-in case list
+"""
+import re, sys
+
+CASES = [
+    ("eff-bit-census.txt", "structures/ui-paint-order-key.md",
+     [14709, 2338, 2657, 1399, 8315, 0.468, 0.144]),
+    ("plateau-census.txt", "structures/ui-resting-pose.md",
+     [15493, 3807, 24.57, 50.2]),
+    ("rotation-toplevel-census.txt", "structures/ui-keyframe-rotation.md",
+     [2152, 13.89]),
+    ("eff-bit-alpha-test.txt", "structures/ui-paint-order-key.md",
+     [55.52, 33.66, 52.52, 30.17, 76.5, 64.1]),
+]
+NUM = re.compile(r"\d[\d   ,]*\.?\d*")
+
+def nums(text):
+    out = set()
+    for m in NUM.finditer(text):
+        try: out.add(float(re.sub(r"[   ,]", "", m.group())))
+        except ValueError: pass
+    return out
+
+def main():
+    bad = 0
+    for dfile, mfile, figs in CASES:
+        D = nums(open(f"docs/re/data/{dfile}", encoding="utf-8").read())
+        M = nums(open(f"docs/re/{mfile}", encoding="utf-8").read())
+        for f in figs:
+            in_d = any(abs(f - x) < 0.011 for x in D)
+            in_m = any(abs(f - x) < 0.051 for x in M)   # the doc may round
+            if not (in_d and in_m):
+                bad += 1
+                print(f"  CHECK {dfile} / {mfile}: {f} data:{in_d} doc:{in_m}")
+    print(f"{sum(len(c[2]) for c in CASES)} figures checked, {bad} to look at")
+    return 1 if bad else 0
+
+if __name__ == "__main__":
+    sys.exit(main())