[feat](trx-ftx): gate FT2 support behind ft2 feature flag, disabled by default

FT2 decoder implementation, protocol constants, server decoder tasks,
background decode, and registry entry are now conditional on the ft2
feature. Lightweight types (enum variants, commands, state fields) remain
unconditional to avoid cascading cfg noise in macros and serde.

Enable with: cargo build -p trx-server --features ft2

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Stan Grams <sjg@haxx.space>

src/trx-protocol/Cargo.toml

@@ -7,6 +7,10 @@ name = "trx-protocol"
 version.workspace = true
 edition = "2021"
 
+[features]
+default = []
+ft2 = []
+
 [dependencies]
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }