Ignore voice-lab WAVs; the findings and code stay tracked

The Author voice lab committed 45 sample renders (25 MB) plus the six-file
reference chain. Those are binaries and every one of them is regenerable from
the scripts sitting beside them, so tracking them buys nothing and costs a
git-ls-files full of audio.

Tracked deliberately:
  backup/author_voice_lab/FINDINGS.md      the measurements and the reasoning
  backup/author_voice_lab/*.json           results + cliff verification
  backup/author_voice_lab/scripts/*.py     everything that produces the audio
  backup/author_voice_matrix/*.pt          the vmatrix itself (62 KB, load-bearing)
  backup/author_voice_matrix/manifest.json SHA-256 + provenance per file
  backup/author_voice_matrix/RESTORE.md    how to put it back

Ignored:
  backup/author_voice_lab/wav/  wav_cliff/   25 MB of renders
  backup/author_voice_matrix/**/*.wav        the reference chain, 25 MB

Note this only stops future tracking. The 45 WAVs are already in history from
the earlier commit, so they stay in the tree until removed with
`git rm -r --cached backup/author_voice_lab/wav backup/author_voice_lab/wav_cliff`
and committed -- which is deliberately left as a separate, explicit step rather
than folded in here, since it changes what a checkout contains.

.gitignore

@@ -50,3 +50,10 @@ umbraaudio_package.zip
 
 # SQLite project databases are runtime state, never source
 *.db
+
+# Voice-matrix lab samples: large binaries, regenerable from the scripts beside
+# them (backup/author_voice_lab/scripts/). The findings, the measurement JSON
+# and the code stay tracked.
+/backup/author_voice_lab/wav/
+/backup/author_voice_lab/wav_cliff/
+/backup/author_voice_matrix/**/*.wav