Decision: drill pulses pass through rubble and fluidise it (t92)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/DESIGN.md

@@ -535,6 +535,12 @@ above, including:
   opening the file from disk is dropped; determinism is re-baselined once and becomes the same on
   any number of cores; as fast as possible, measured at every step. Grain movement and heat go
   across a worker pool first; moving the whole simulation into its own worker is decided after.
+- **Drill pulses pass through rubble and fluidise it (t92, 2026-10-28).** Loose material used to
+  absorb about 12% of a pulse per grain, so a modest pile blocked the drill completely. The pulse
+  is tuned to make solid rock resonate and loose grains can't hold that vibration, so now loose
+  material costs about 3% per grain, and the pulse fluidises it (grains kick, piles slump and
+  flow, pockets drain), with a little dust. All damping is counted per grain travelled
+  (attenuation is per distance in real physics), retuned so rock bores feel the same as before.
 
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