proto: no_std hygiene for the fmt/alloc imports and the filter math

Bring the proto tree closer to a std+alloc-only posture, behavior unchanged on
every decision path:

- Sweep the remaining std::fmt and std::collections imports over to core::fmt
  and alloc::collections across the wire codecs and their tests. Subsystem files
  that shadow the core name with a child core module use the leading-colon
  ::core::fmt form. Imports only.

- Replace the two f64::powi calls (bloom false-positive-rate, FMP backoff timer)
  with a shared core-only square-and-multiply helper in proto/math, bit-identical
  to std::powi (a guard test pins this against every exponent the codecs reach),
  and route the diagnostic estimated-count natural log through libm::log. The FPR
  reject decision and the backoff timer are bit-for-bit unchanged; only the
  debug-only count estimate may differ by at most one ULP.

Cargo.toml

@@ -17,6 +17,7 @@ secp256k1 = { version = "0.30", features = ["rand", "global-context"] }
 sha2 = "0.10"
 hkdf = "0.12"
 ring = "0.17"
+libm = "0.2"
 rand = "0.10.1"
 crossbeam-channel = "0.5"
 thiserror = "2.0"