Correct the pocketsphinx instruction and document the apt/pip boundary

Debian ships the pocketsphinx tools and library but not the Python bindings,
so "apt install pocketsphinx" did not give a working engine. It comes from
pip like the others.

Also explains in the README why no recogniser can be a package dependency:
Policy forbids anything in the archive from requiring software outside it,
and a postinst that fetched from PyPI would break offline and reproducible
installs. Notes that a pip --user install still works with a .deb-installed
bandsaunter, since the user site directory is on the system interpreter's
path -- checked against the built package rather than assumed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -59,7 +59,7 @@ be built.
 | **required** | PyYAML | `python3-yaml` | `python3-pyyaml` | `python-yaml` | settings file and profiles |
 | *recommended* | eSpeak NG | `espeak-ng` | `espeak-ng` | `espeak-ng` | clearer spoken timestamps |
 | *optional* | rtl-sdr tools | `rtl-sdr` | `rtl-sdr` | `rtl-sdr` | `rtl_test` and friends for diagnosis |
-| *optional* | faster-whisper | pip only | pip only | AUR | transcribing speech to text |
+| *optional* | a speech recogniser | **pip only** | **pip only** | AUR | transcribing speech to text |
 | *optional* | Matplotlib | `python3-matplotlib` | `python3-matplotlib` | `python-matplotlib` | nothing yet; reserved for plots |
 
 Two notes on the optional ones:
@@ -69,9 +69,9 @@ timestamps come from a built-in formant synthesiser, so that feature works on
 a machine with nothing else installed. With it they are clearer and render
 about three times faster.
 
-**Speech recognition is not packaged for Debian** and can only come from pip.
+**No speech recogniser is packaged for Debian.** `faster-whisper`, `vosk` and
-Transcription is off by default and says so plainly when no recogniser is
+the pocketsphinx Python bindings are all absent from the archive, so
-present, so this never blocks an install:
+transcription can only be installed with pip:
 
 ```bash
 pip install faster-whisper     # best on radio audio, ~120 MB
@@ -79,6 +79,33 @@ pip install vosk               # ~10 MB plus a 40 MB model, weaker on noise
 bandsaunter transcribe --engines
 ```
 
+That is why transcription is `Suggests:` rather than `Depends:` in the
+package. Debian Policy forbids anything in the archive from requiring
+software outside it, and a `postinst` that reaches out to PyPI would break
+offline and reproducible installs — so a package simply cannot pull these in.
+Transcription is therefore off by default and reports plainly when no
+recogniser is present, rather than the install failing or the feature
+appearing broken.
+
+Mixing the two is nonetheless fine here. Modern Debian marks the system
+Python as externally managed (PEP 668), so a pip install lands in your user
+site directory:
+
+```bash
+pip install --user faster-whisper      # ~/.local/lib/python3.x/site-packages
+```
+
+which is on `sys.path` for the system interpreter. A bandsaunter installed
+from the `.deb` into `/usr/lib/python3/dist-packages` picks it up with no
+further configuration — verified, not assumed. A virtual environment works
+too, as long as bandsaunter runs inside it.
+
+If you would rather keep everything under apt, the only route is packaging a
+recogniser for Debian yourself. That is a real undertaking for whisper: it
+would mean packaging ctranslate2, tokenizers, onnxruntime and their
+dependencies, several of which are large C++ or Rust projects. It is why
+none of them are there.
+
 ### Other distributions
 
 ```bash

bandsaunter/transcribe.py

@@ -43,7 +43,9 @@ _ENGINE_NOTES = {
     "whisper": "pip install openai-whisper",
     "whisper-cli": "whisper.cpp, no Python dependencies",
     "vosk": "pip install vosk  (small, weaker on noisy audio)",
-    "pocketsphinx": "apt install pocketsphinx  (tiny, poor on radio audio)",
+    # Debian ships the pocketsphinx tools and library but not the Python
+    # bindings, so apt does not give a working engine here.
+    "pocketsphinx": "pip install pocketsphinx  (tiny, poor on radio audio)",
 }