Stop AudioChannels and SampleRate fabricating a value for Unknown

Three copies of the same two mappings existed. The canonical accessors
returned 6 channels and 48000 Hz for Unknown; a third copy in diag.rs
returned 0. The honest one was the copy.

A plausible wrong answer is worse than an obvious one. Six channels at
48 kHz is indistinguishable from a real 5.1 track, so every caller became
responsible for remembering to check the variant first — and this crate
walked into exactly that: the json:// sink reported a confident 5.1 for
audio whose neighbouring fields said "unknown". That was fixed at the call
site earlier in this audit; this fixes it at the source.

The accessors now return 0, which is what both in-crate call sites already
coerced Unknown to by hand, so their guards are gone and the behaviour is
unchanged. Zero is also obviously wrong if it ever reaches output, where
six is not.

The diag.rs duplicates are deleted rather than corrected — a fourth copy
would have drifted too. Their only caller was a trace line in the same
file, now on the canonical accessors. Their tests moved across and gained
the Unknown case, which is the point: restoring either fabricated value
fails both.

Found by the round-7 correctness agent while fixing the json:// sink; it
flagged the third copy as out of its scope rather than touching it.

src/disc/mod.rs

@@ -1035,7 +1035,14 @@ impl AudioChannels {
             AudioChannels::Surround51 => 6,
             AudioChannels::Surround61 => 7,
             AudioChannels::Surround71 => 8,
-            AudioChannels::Unknown => 6,
+            // 0, not 6. An unknown layout has no channel count, and returning a
+            // plausible one made every caller responsible for remembering to
+            // check the variant first — a trap, and one this crate walked into:
+            // the json:// sink reported a confident 5.1 for audio its own
+            // neighbouring fields called "unknown". 0 is the value Matroska and
+            // the sinks already coerce Unknown to, and unlike 6 it is obviously
+            // wrong if it ever reaches output.
+            AudioChannels::Unknown => 0,
         }
     }
 
@@ -1082,7 +1089,9 @@ impl SampleRate {
             SampleRate::S96 => 96000.0,
             SampleRate::S176_4 => 176400.0,
             SampleRate::S192 => 192000.0,
-            SampleRate::Unknown => 48000.0,
+            // 0.0, not 48000.0 — see AudioChannels::count. A fabricated rate is
+            // indistinguishable from a real one; a zero is not.
+            SampleRate::Unknown => 0.0,
         }
     }