feat(inbox): .pptx extraction — speaker notes, off-slide box, alt-text (stage 2e)

The pptx payload hides where an audience watching the slides does not look. The
extractor surfaces all three regions into the concept text so the stage-2 scan
catches the injection:

- speaker notes (slide.notes_slide.notes_text_frame.text);
- off-slide (off-canvas) text boxes (shape.text_frame.text, position-agnostic);
- image/shape alt-text (cNvPr@descr, read off the XML — python-pptx 1.0.2 has no
  stable public accessor across shape types).

Detach-proof: same visible slide without notes ADMITs. python-pptx imported
lazily (dev/showcase-scoped, not a core dep). Tests 305 -> 310.

tests/inbox_frontend.py

@@ -150,6 +150,38 @@ def _extract_docx_text(fs_path) -> str:
     return "\n".join(parts)
 
 
+def _shape_alt_text(shape) -> str:
+    """Read a shape's alt-text (cNvPr@descr). python-pptx 1.0.2 has no stable
+    public accessor across shape types, so read it off the XML directly."""
+    for element in shape._element.iter():
+        if element.tag.endswith("}cNvPr"):
+            return element.get("descr") or ""
+    return ""
+
+
+def _extract_pptx_text(fs_path) -> str:
+    """Extract text from a ``.pptx``, including the regions an audience watching
+    the slides does not see: speaker notes, off-slide (off-canvas) text boxes, and
+    image/shape alt-text. ``python-pptx`` is imported lazily (dev/showcase-scoped).
+    """
+    from pptx import Presentation
+
+    prs = Presentation(str(fs_path))
+    parts: list = []
+    for slide in prs.slides:
+        for shape in slide.shapes:
+            if shape.has_text_frame and shape.text_frame.text:
+                parts.append(shape.text_frame.text)   # incl. off-slide boxes
+            alt = _shape_alt_text(shape)
+            if alt:
+                parts.append(alt)
+        if slide.has_notes_slide:
+            notes = slide.notes_slide.notes_text_frame.text
+            if notes:
+                parts.append(notes)
+    return "\n".join(parts)
+
+
 def _ingest_regular_file(fs_path, rel_name, bundle, provenance, rejected, *, strict):
     """Dispatch one on-disk file by suffix. ``strict`` raises on an unsupported
     suffix (a top-level drop); a folder walk passes ``strict=False`` to skip it."""
@@ -159,6 +191,8 @@ def _ingest_regular_file(fs_path, rel_name, bundle, provenance, rejected, *, str
         _ingest_csv(rel_name, text, bundle, provenance, rejected)
     elif suffix == ".docx":
         _materialize_text(rel_name, _extract_docx_text(fs_path), "docx", bundle, provenance)
+    elif suffix == ".pptx":
+        _materialize_text(rel_name, _extract_pptx_text(fs_path), "pptx", bundle, provenance)
     elif suffix in _TEXT_SUFFIXES:
         text = Path(fs_path).read_text(encoding="utf-8", errors="replace")
         _materialize_text(rel_name, text, suffix.lstrip("."), bundle, provenance)