Core ride logic, FTMS client, FIT encoder and probe CLI

Adds backing state for Resistance and Erg control modes, which had no
value to hold and so could never satisfy FR-4.3/FR-4.6.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

crates/fit/Cargo.toml

@@ -8,3 +8,11 @@ license.workspace = true
 bikecontrol-core = { workspace = true }
 thiserror = { workspace = true }
 chrono = { workspace = true }
+serde = { workspace = true }
+serde_json = { workspace = true }
+
+[dev-dependencies]
+# Independent third-party FIT *decoder* (MIT). Test-only: we encode with our own
+# writer and decode with someone else's parser, which is a far stronger check
+# than round-tripping through our own code.
+fitparser = "0.11"