Turn a release-only slice panic into an error, and stop calling 32 kHz 48 kHz

Four round-6 findings.

FileSectorSource::read_sectors guarded its output buffer with a
debug_assert, which is compiled out in release — so an undersized buffer
panicked with 'range end index out of range' instead of returning an
error, out of a public SectorSource impl where the length is caller input.
Drive::read_fua already carries this exact guard, with a comment recording
the same panic being fixed there, and PrefetchedSectorSource has a
regression test for the same case; this impl had been given neither. The
new test is red in release for precisely the predicted reason: 'range end
index 8192 out of range for slice of length 2049'.

parse_track's sample-rate ladder ended in an unconditional S48, so any
SamplingFrequency below 44100 was recorded as 48 kHz. A 32000 Hz AC-3 or
DTS track is legal and common in broadcast-sourced content, and the wrong
rate then propagated into the reconstructed AudioStream. Anything below
the lowest mapped rate is now Unknown, which is what the crate's canonical
SampleRate::from_hz already returned — the ladder disagreed with it. The
ladder itself stays, because the MKV element is a float and wants
tolerance rather than exact equality.

shim_open_exclusive used the mach port from IOMainPort without checking
the return; on failure the port is left untouched and every IOKit call
below ran against an uninitialised value. shim_list_drives in the same
file does check it.

build.rs treated cc and ar as successful if the process merely SPAWNED, so
a genuine compile error in the macOS C shim produced no object file and
surfaced later as an unexplained link failure against a missing symbol.
The shim is macOS-only and is neither linted nor compiled on the other two
platforms, so a mistake in it has exactly one chance to be noticed.

The last two have no test: one needs IOMainPort to fail, the other needs a
deliberately broken C shim, and neither is reachable from the test
harness. Both mirror a correct sibling in the same file, which is the
evidence available.

src/scsi/macos_shim.c

@@ -244,7 +244,10 @@ int shim_open_exclusive(const char *bsd_name) {
     usleep(500000);
 
     mach_port_t mp;
-    IOMainPort(0, &mp);
+    // Check the return before using the port. On failure IOMainPort leaves `mp`
+    // untouched, so every IOKit call below would run against an uninitialised
+    // mach port. shim_list_drives does check it; this path did not.
+    if (IOMainPort(0, &mp) != kIOReturnSuccess) return -1;
 
     io_service_t svc = find_bdsvc_by_bsd_name(mp, bsd_name);
     if (!svc) {