docs: pin the README robustness GIF to a commit SHA so it renders pre-merge

The inline GIF used a master raw URL, which 404s until the file is on master
(broken even when viewing the branch). Pin it to the commit SHA that already
contains the GIF on the remote: an absolute URL that renders immediately on the
branch, keeps README.md and docs/README.md identical, and works on PyPI. (After
this PR merges the file is on master too; the SHA URL remains valid.)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013KvuS9HKbnZAwwFJyBkyHc

Author: claude

README.md

@@ -306,7 +306,7 @@ disturbance gain on every system, **practically significantly on 10/13** — mos
 LQR leaves a sharp resonant peak — each at a documented nominal-cost price:
 
 <p align="center">
-    <img alt="H-infinity game vs LQR under worst-case disturbance (inverted pendulum)" src="https://raw.githubusercontent.com/krichelj/PyDiffGame/master/benchmarks/results/robust_inverted_pendulum.gif" width="860"/>
+    <img alt="H-infinity game vs LQR under worst-case disturbance (inverted pendulum)" src="https://raw.githubusercontent.com/krichelj/PyDiffGame/bbd010f15ee13adc2bba5e7b99e1a3ecc0238583/benchmarks/results/robust_inverted_pendulum.gif" width="860"/>
 </p>
 
 Left: the pendulum-angle response to the worst-case disturbance (H∞ roughly halves

docs/README.md

@@ -306,7 +306,7 @@ disturbance gain on every system, **practically significantly on 10/13** — mos
 LQR leaves a sharp resonant peak — each at a documented nominal-cost price:
 
 <p align="center">
-    <img alt="H-infinity game vs LQR under worst-case disturbance (inverted pendulum)" src="https://raw.githubusercontent.com/krichelj/PyDiffGame/master/benchmarks/results/robust_inverted_pendulum.gif" width="860"/>
+    <img alt="H-infinity game vs LQR under worst-case disturbance (inverted pendulum)" src="https://raw.githubusercontent.com/krichelj/PyDiffGame/bbd010f15ee13adc2bba5e7b99e1a3ecc0238583/benchmarks/results/robust_inverted_pendulum.gif" width="860"/>
 </p>
 
 Left: the pendulum-angle response to the worst-case disturbance (H∞ roughly halves