Scaffold Ergaenzung/ for supplementary practice books

Adds structure for the three Hueber Deutsch üben books (Wortschatz &
Grammatik, Hören & Sprechen, Lesen & Schreiben A1), kept separate from
the Menschen A1.1 lesson files since they're organized by topic/unit
rather than a shared lesson curriculum.

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

README.md

@@ -25,6 +25,12 @@ A1.1/
   INDEX.md        ← one-line summary of every lesson so far
 A1.2/             ← added once I move on to the next book
   ...
+Ergaenzung/       ← supplementary practice books (see below)
+  wortschatz-grammatik-a1/
+  hoeren-sprechen-a1/
+  lesen-schreiben-a1/
+  _template.md
+  INDEX.md
 scripts/
   transcribe.py   ← local ASR fallback (faster-whisper) for audio with no
                      official transcript
@@ -85,3 +91,32 @@ given track, e.g. supplementary listening material):
 
 Copy the `A1.1/` folder structure (minus content) into a new `A1.2/` folder
 and repeat the process.
+
+## Supplementary practice books (`Ergaenzung/`)
+
+Alongside *Menschen*, I also use three Hueber *Deutsch üben* practice
+books: **Wortschatz & Grammatik A1**, **Hören & Sprechen A1**, and **Lesen
+& Schreiben A1**. Unlike *Menschen*, these aren't organized lesson-by-lesson
+against a shared story/curriculum — each is structured around its own
+topics/units (grammar points for Wortschatz & Grammatik, skill-practice
+units for the other two). Forcing them into the `A1.1/lektion-XX.md`
+structure would misrepresent that, so they get their own space instead:
+
+```
+Ergaenzung/
+  wortschatz-grammatik-a1/einheit-XX.md
+  hoeren-sprechen-a1/einheit-XX.md
+  lesen-schreiben-a1/einheit-XX.md
+  _template.md   ← template for these files
+  INDEX.md       ← per-book tables mapping each unit to Menschen Lektion(en)
+```
+
+Each `einheit-XX.md` is scoped like a lesson file (vocab/grammar covered,
+exercise summary, audio transcript if applicable, answer key highlights,
+notes) but adds a **"Relates to Menschen Lektion(en)"** field so a tutor
+session can tell whether content is reinforcing what's already been covered
+in *Menschen* or introducing something new/out-of-scope.
+
+Workflow for adding a unit: same as adding a *Menschen* lesson (get the
+pages as PDF/photo, Claude summarizes into `einheit-XX.md` using
+`Ergaenzung/_template.md`, review, update `Ergaenzung/INDEX.md`, commit).