mux/meta: preserve color_space round-trip in m2ts metadata

MetaStream::Video dropped color_space on from_title/to_title, hardcoding
BT.709 on the way back. HDR titles (BT.2020) lost their color metadata.

Add a color_space field, populate it in from_title, and use it in
to_title. For pre-0.30.7 metadata that has no color_space, derive it from
the preserved hdr field (all HDR formats are BT.2020, SDR is BT.709).

Adds ColorSpace::id() + FromStr for serialization round-trip.

src/disc/mod.rs

@@ -779,6 +779,22 @@ impl ColorSpace {
             ColorSpace::Unknown => "",
         }
     }
+
+    const ALL_CS: &[(&'static str, ColorSpace)] = &[
+        ("bt709", ColorSpace::Bt709),
+        ("bt2020", ColorSpace::Bt2020),
+        ("unknown", ColorSpace::Unknown),
+    ];
+
+    /// Compact identifier for serialization (round-trips via `FromStr`).
+    pub fn id(&self) -> &'static str {
+        for (id, v) in Self::ALL_CS {
+            if v == self {
+                return id;
+            }
+        }
+        "unknown"
+    }
 }
 
 impl std::fmt::Display for ColorSpace {
@@ -787,6 +803,24 @@ impl std::fmt::Display for ColorSpace {
     }
 }
 
+impl std::str::FromStr for ColorSpace {
+    type Err = ();
+    fn from_str(s: &str) -> std::result::Result<Self, ()> {
+        for (id, v) in ColorSpace::ALL_CS {
+            if *id == s {
+                return Ok(*v);
+            }
+        }
+        // Also accept display names (e.g. "BT.2020").
+        for (_id, v) in ColorSpace::ALL_CS {
+            if ColorSpace::name(v) == s {
+                return Ok(*v);
+            }
+        }
+        Ok(ColorSpace::Unknown)
+    }
+}
+
 // ─── FromStr impls — single source of truth via ALL_* arrays ───────────────
 //
 // Each enum defines a const array of (str, variant) pairs. Display, FromStr,