fix: Patch coarsetime for MIPS AtomicU64 support

- Vendor coarsetime crate with portable-atomic fallback
- Use portable-atomic on targets without 64-bit atomics
- Patch crates.io to use local coarsetime
- Fixes MIPS build failure (AtomicU64 missing)

Cargo.toml

@@ -9,3 +9,6 @@ lto = true
 codegen-units = 1
 panic = "abort"
 strip = true
+
+[patch.crates-io]
+coarsetime = { path = "third_party/coarsetime" }