Migrate the FMP discovery decision logic out of the async handlers into
synchronous, runtime-agnostic sans-IO state machines owned by the protocol
structs, with I/O pushed to the edges. Pulls the full decision surface into a
pure core (backoff, rate-limit, planners, response routing), consolidates the
tests into a per-module tree with a shared crate testutil, and injects a u64
wall-clock so the core is free of Instant and std time.
Also brings the module toward no_std+alloc: the four discovery maps use
alloc::collections::BTreeMap (HashMap's RandomState is std-only), Arc is spelled
alloc::sync::Arc, the backoff-reset log lives in the shell (the core returns the
cleared count so observability stays out of the pure core), and the crate root
names alloc directly. The one remaining tether is ProtocolError's
std::error::Error coupling in the wire codec.
First subsystem of the broader sans-IO refactor; establishes the extraction
patterns and conventions carried forward to the remaining protocols.
