fix(callcenter::role_keys): const { assert!() } for const-only invariant (clippy::assertions_on_constants)

CI clippy (`-D warnings`) caught a const-only `assert!` on the
SAVANT_SLICE_END <= VSA_DIMS invariant inside the
`savant_zone_fits_in_smb_headroom` test. Both operands are
compile-time constants, so the right shape is `const { assert!(..) }`
— compile-time check, no runtime work, no lint.

Lines 170-172 (SUBJECT_KEY.slice_end ≤ SAVANT_SLICE_START etc.) use
the same pattern but aren't flagged by clippy 1.95 (struct field
accesses on const RoleKey values aren't const-evaluated by this
lint); leaving them as runtime asserts pending future clippy upgrade.

https://claude.ai/code/session_017gZ6sPRXYPj5n7uJ7NBtRv

Author: claude

crates/lance-graph-contract/src/callcenter/role_keys.rs

@@ -120,7 +120,7 @@ mod tests {
         // [14_096 .. 16_384) (= 2288 dims) as headroom. We claim 2250 of
         // those 2288 dims; 38 remain.
         assert_eq!(SAVANT_SLICE_END, 16_346);
-        assert!(SAVANT_SLICE_END <= VSA_DIMS);
+        const { assert!(SAVANT_SLICE_END <= VSA_DIMS) };
         assert_eq!(VSA_DIMS - SAVANT_SLICE_END, 38, "headroom remaining");
     }