spec, test_util: specification quote registry, spec guards, test-util fixtures

libfreemkv::spec holds the verbatim specification quotes the code
cites, with guards tying each governed site to its quote. A test-util
feature exposes shared fixtures to downstream tests. M2TS output
derives DTS for reordered video and rounds to whole 90 kHz ticks.

Cargo.toml

@@ -51,6 +51,9 @@ rip = [
     "dep:socket2",
     "dep:rustix",
 ]
+# `test-util`: `libfreemkv::test_util` fixtures for consumers' tests (dev-dependency
+# only; never in a release build). Implies the rip tree the fixtures build on.
+test-util = ["rip"]
 
 [dependencies]
 serde = { version = "1", features = ["derive"], optional = true }