port: withdraw my own "two stems" reading of a voice region, and stop summing silence

The Decoder asked me to decode a voice region's leading chunk -- it has no XMA1
decoder in its container -- and the decoder run refuted a claim of mine that it
had already adopted into `docs/re/structures/voice-region-leading-chunk.md`.

I wrote that a region's two equal-length chunks are HANDOFF Q10's decoded
two-stem shape. Equal duration was a SHAPE match and I carried the music census
across on the strength of it. The content does not support it:

  S00A chunk 2 is DIGITAL SILENCE -- 4497300 samples, peak -inf.
  ADV  chunk 2 is 0.60x chunk 1, best-fit scalar, residual 26.8 dB below the
       target: about 95% of its energy is a -4.4 dB copy of the first chunk.

That cost real level. Summing chunk 1 with silence at 1/n put S00A's dialogue
6.02 dB down for nothing -- the exported file peaked at -16.2 dBFS against a
source chunk peaking at -4.2. `export_voice` now drops a digitally silent chunk
before the sum, which is arithmetic and not a judgement about content.

WHAT ADV'S NEAR-DUPLICATE SECOND CHUNK IS REMAINS OPEN AND IT IS STILL SUMMED.
Whether the game plays both is a decoding question, 26.8 dB of residual is not
nothing, and dropping a chunk because it correlates with another would be
answering it.

The leading chunk, answered as far as a measurement goes: ADV region + 1392, 394
packets, 84.553 s, stereo 48 kHz, peak -2.48 dBFS, 6 silent gaps over 0.4 s
totalling 45.3 s -- 54% silence, the same duty cycle as the full-length chunks.
Speech-structured, so not a header and not padding. "Cutscene or mission" is an
identification and this agent has no ears and no oracle; envelope correlation
peaks at 0.768 at the last lag in the search range, which is where a statistic
lands when it has found nothing, and it is not an answer.

Not taken yet, and said so in BLOCKED: the discriminator should be
`bank_header_len`, not a duration tie. This exporter never used `riffs.len()`, so
it already handles both of the Decoder's cases, but a tie is an observation and
`bank_header_len` is decoded. It switches when `c1f3608` reaches `main`;
`sylpheed-formats` is a path dependency and merging another agent's topic branch
is not the port's to do.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N7FiFFFwbvG2uxdcEh8HyF

crates/sylpheed-export/examples/voice_chunks.rs

@@ -33,7 +33,14 @@ fn main() {
                 .output()
                 .unwrap();
             let dur = String::from_utf8_lossy(&out.stdout).trim().to_string();
-            let _ = std::fs::remove_file(&w);
+            if std::env::var("KEEP_WAV").is_ok() {
+                let keep = std::path::Path::new(&std::env::var("KEEP_WAV").unwrap())
+                    .join(format!("{movie}_chunk{i}.wav"));
+                let _ = std::fs::rename(&w, &keep);
+                println!("      kept -> {}", keep.display());
+            } else {
+                let _ = std::fs::remove_file(&w);
+            }
             println!("   chunk {i}: {} bytes -> {dur} s", r.len());
             let _ = std::fs::remove_file(&p);
         }