Harden EPLAN import: format detection, trim SDK, docs (M4)

- EdzArchive checks the archive magic up front: gives a clear message for
  zip-format .edz (not yet supported) and unrecognised data, instead of an
  opaque 7z decode error.
- Trim the vendored LZMA SDK headers to the decode closure actually used
  (removes 21 unused encoder/multithread/Xz/Aes headers; 18 .c + 18 .h remain).
- Add sources/import/edz/README.md documenting the feature, the data mapping
  and the bundled SDK.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sources/import/edz/README.md

@@ -0,0 +1,52 @@
+# EPLAN `.edz` part import
+
+Imports a part from the **EPLAN Data Portal** (`.edz`) into a QElectroTech
+element collection. The EPLAN Data Portal is the de-facto parts catalogue for
+electrical CAD (ifm, Phoenix Contact, Siemens, Rittal, Weidmüller, Schneider…),
+and components ship as `.edz`; this lets a QET user drop one straight into a
+project.
+
+## Using it
+
+Right-click a writable collection folder in the elements panel →
+**Import an EPLAN part (.edz)…** → choose the file. The generated element is
+written into that folder and the panel reloads.
+
+## What is imported
+
+A `.edz` is a 7-Zip archive containing a part definition (`*.part.xml`), an EPLAN
+macro (`*.ema`), a product image and metadata. Only the **portable** part data is
+used:
+
+- **Terminals** — each `<functiontemplate>` with a physical connection
+  designation becomes a QET terminal, labelled from its connection description.
+- **Element information** — manufacturer, order number, designation and comment
+  fill the QET element-information fields used by the BOM / nomenclature.
+- **Localized names** — one `<name lang="xx">` per language the part provides, so
+  the element labels itself in any QET UI language.
+- **Symbol** — a generic symbol is generated (a body rectangle with one
+  west-facing terminal per pin). The EPLAN `.ema` macro geometry is **not**
+  reproduced: it is tokenised EPLAN PXF that references EPLAN's own symbol
+  libraries. A generic symbol is sufficient for wiring and BOM; swap in a nicer
+  symbol in the element editor afterwards if desired.
+
+## Structure
+
+| File | Role |
+|------|------|
+| `edzarchive.*` | Extract the `.edz` to a temp dir; locate `part.xml` |
+| `edzsevenzip.*` | 7-Zip extraction via the bundled LZMA SDK (no external 7-Zip) |
+| `edzpart.*` | Parse `part.xml` into a portable model |
+| `edzelementbuilder.*` | Build the `.elmt` element from the model |
+| `edzimporter.*` | Orchestrate the above and write into a collection folder |
+| `lzma/` | Vendored public-domain LZMA SDK (decode-only subset) |
+
+The non-UI classes are deliberately decoupled from the widget so they can be
+tested headless.
+
+## Bundled LZMA SDK
+
+`lzma/` contains the decode-only subset of Igor Pavlov's **LZMA SDK** (public
+domain — see `lzma/LZMA-SDK-LICENSE.txt`), so a `.edz` can be unpacked without an
+external 7-Zip install. Only 7-Zip-format `.edz` files are supported; zip-format
+packages are detected and reported but not yet handled.