verification: Coulomb friction between sliding blocks

An oracle case of a block pressed on another and slid with static and kinetic coefficients
(mu_k N carried once slipping), and the docs of the friction formulation; ROADMAP M7 and
parity updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UhhZMBTviDQxZvwnyP89fo

tests/oracle/nl-friction-sliding-blocks/fixture.json

@@ -0,0 +1,17 @@
+{
+  "note": "FieldCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "balance": {
+      "band": 1e-9,
+      "value": -0.999999977621409
+    },
+    "kinetic": {
+      "band": 2.0000000000000003e-10,
+      "value": 0.199999793528717
+    },
+    "slipping": {
+      "band": 9.000000000000001e-9,
+      "value": 9.0
+    }
+  }
+}