fix: sched round-robin, cap slot OOB panic, clippy, make ci

- RunList -> intrusive FIFO, no starvation, no silent drop
- cap table sized to CapSlot range (kani caught OOB panic)
- snake food color test uses const
- kani cfg lint, RUST_MIN_STACK for big kernel struct in tests

.cargo/config.toml

@@ -1,3 +1,7 @@
 [target.riscv64gc-unknown-none-elf]
 runner = "qemu-system-riscv64 -machine virt -nographic -bios default -kernel"
 rustflags = ["-C", "panic=abort"]
+
+[env]
+# Kernel tables are large fixed arrays; tests build `Kernel` by value.
+RUST_MIN_STACK = "33554432"