BAL - Bitcoin After Life Electrum plugin (v0.2.8)

Behavior-preserving refactor of the original BAL plugin with clean separation
of business logic from the PyQt GUI.

Layout:
  bal/core/   GUI-free logic (util, plugin_base, heirs, will, willexecutors)
  bal/gui/qt/ PyQt6 presentation (theme, common, widgets, calendar, dialogs,
              lists, window, plugin)
  bal/qt.py   Qt entry-point shim (works as internal and external zip plugin)
  bal/manifest.json  standard-conforming metadata

Tooling:
  build_zip.py            deterministic, zipimport-friendly archive builder
  tests/smoke_test.py     imports + behavior regression test
  tests/external_zip_test.py  reproduces Electrum's external-zip loading

Targets Electrum 4.7.2 + PyQt6. Logic kept byte-identical where possible.

bal/core/__init__.py

@@ -0,0 +1,21 @@
+"""
+bal.core
+========
+
+Pure business-logic layer of the Bitcoin After Life (BAL) Electrum plugin.
+
+Everything in this sub-package MUST stay completely free of any GUI / Qt
+imports.  The rule of thumb is:
+
+    * ``bal.core``      -> "what the plugin does" (inheritance rules, building
+                           and validating transactions, talking to
+                           will-executor servers, persistence helpers).
+    * ``bal.gui``       -> "how it looks" (Qt widgets, dialogs, list views).
+
+Keeping the two apart is the main motivation behind this rewrite: the original
+code mixed transaction-building logic and presentation inside a single
+4000-line ``qt.py`` module, which made the delicate Bitcoin logic hard to audit.
+
+No behaviour is changed with respect to the original plugin; the code has only
+been reorganised and documented.
+"""