The belly is not armoured (R-254)

R-95 made losing a wing change the animal. R-96 took the hide off the
wing so losing one was possible. Separately each was right and each was
measured. Together they gave the party a soft target and deleted it the
moment the party hit it:

  winged     6/20 ranged faces land on armour 0 (a bare wing)
  quadruped  0/20

A service round on borough ground averages 3.75 after halving — 3.75
through a bare wing, 0 through hide 7 — so grounding the creature took
the party's chance of winning from 44.9% to 1.4%, while its own tactics
told the GM that grounding it was the fight they could actually win.

Found by measuring something else. Every aimed-shot configuration made
the party worse, and the most aggressive was the worst: aiming at a wing
at -20% grounds it in 99% of fights and HALVES the win rate to 10.1%.
Buying the objective reliably was the fastest way to lose, which only
makes sense if the objective is a trap. So no aimed-shot rule; the
problem was never the lack of one.

It also retires the "+31 points" I reported for grounding at 1.7.3. That
was win-rate-if-grounded against win-rate-if-not — selection bias, since
the parties that grounded it were the ones shooting well. Forcing the
grounding gives the causal value and it was -43. A correlation of +31
and a causation of -43 out of the same mechanic.

The fix: a heraldic beast is armoured the way it is drawn, across the
back and the flanks. On its belly half that hide is no longer in the
way, so exposedHideFor halves natural armour while system.grounded is
set. Hide 7 becomes 3.

                      flying   grounded   worth
  on borough ground    45.3%     51.7%    +6.4
  off borough ground   90.6%     97.9%    +7.3

Modest deliberately. Grounding should help, not decide — the borough is
still the real answer. Losing its damage modifier was worth +26.5 and
would have made the wing the whole fight.

check-anatomy gains a sixth property: a flyer must not be harder to hurt
on the ground than in the air. The first version probed at a single
4-point round and declared the fix broken, because 4 sits inside the one
band where a bare wing beats a halved hide — below 5 damage the creature
really is better off grounded, above it much worse. Integrating across
1-12 gives 2.83 flying against 3.75 grounded and reads it correctly.
Negative-tested against 1.7.8's behaviour, which it names.

The lethality baseline is unchanged: its simulation never grounds
anything, so system.grounded never comes up there.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/REVIEW_LOG.md

@@ -5253,3 +5253,62 @@ the whole nine-guard suite        2.55s
 Ten times the statistical work for one and a half seconds. The guard had been carrying a
 tolerance of six points, and then an exact comparison against figures that could move
 eleven, for want of a measurement that cost less than the time to read this paragraph.
+
+### R-254. Grounding it was a forty-three point mistake
+
+Two commits in a row shipped a defect that neither of their guards could see, and it
+took an unrelated question — what would an aimed shot at −30% be worth? — to find it.
+
+R-95 made losing a wing change the animal. R-96 took the hide off the wing so that
+losing one was possible at all. Separately each was right and each was measured. Together
+they gave the party a soft target and then **deleted it the moment the party hit it**:
+
+```
+winged     6/20 ranged faces land on armour 0 (a bare wing)
+quadruped  0/20
+```
+
+A service round on borough ground averages 3.75 after halving: 3.75 through a bare wing,
+0 through hide 7. So putting the creature on the ground took the party's chance of
+winning from **44.9% to 1.4%** — and its own tactics tell the GM that grounding it is
+"the fight the agents can actually win".
+
+The aimed shot measurement is what exposed it. Every configuration made the party worse,
+and the worst was the most aggressive: aiming at a wing at −20% grounds the creature in
+99% of fights and **halves** the win rate to 10.1%. Buying the objective reliably was the
+fastest way to lose. That only makes sense if the objective is a trap, and it was.
+
+It also retires a number I reported at 1.7.3: that grounding was "worth +31 points",
+measured as win-rate-if-grounded against win-rate-if-not. That was selection bias —
+parties that managed to ground it were the ones shooting well. Forcing the grounding
+reveals the causal value, and it was deeply negative. A correlation of +31 and a
+causation of −43 out of the same mechanic is worth remembering the next time one of
+these numbers looks encouraging.
+
+**The fix.** A heraldic beast is armoured the way it is drawn: across the back and the
+flanks. Down on its belly, half that hide is no longer between you and it, so
+`exposedHideFor` halves natural armour while `system.grounded` is set. Hide 7 becomes 3,
+which restores something an ordinary round can get through without pretending a grounded
+animal is harmless.
+
+| | flying | grounded | grounding is worth |
+|---|---|---|---|
+| on borough ground | 45.3% | 51.7% | **+6.4** |
+| off borough ground | 90.6% | 97.9% | **+7.3** |
+
+Modest on purpose. Grounding it should help, not decide; the borough is still the real
+answer. Of the options measured, losing its damage modifier entirely would have been
+worth +26.5 and made the wing the whole fight.
+
+**The guard, and the two mistakes in building it.** check-anatomy now measures what an
+average round gets through before and after, and fails a flyer that is harder to hurt on
+the ground than in the air. The first version probed at a single 4-point round and
+reported that the fix had failed — because 4 is inside the one narrow band where a bare
+wing beats a halved hide. Below 5 damage the creature really is better off on the ground;
+above it, much worse. Integrating across the range 1–12 gives 2.83 flying against 3.75
+grounded and reads the design correctly.
+
+It is also deliberately one-sided: it catches grounding being a setback, not grounding
+being too generous. Stripping the hide to zero passes it. That is a balance question and
+this is a correctness guard, and conflating the two is how the six-point tolerance in
+check-lethality came to hide a real change for four commits.