UnitPackage and config loading

[withtwoemms]

As an application developer integrating ucon
I want to define domain-specific units in TOML config files
So I can extend ucon at application startup without modifying library code or using global mutable state

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Acceptance Criteria

• GIVEN a valid TOML file with unit definitions
  WHEN load_package(path) is called
  THEN a frozen UnitPackage object must be returned.

• GIVEN a UnitPackage with unit definitions
  WHEN graph.with_package(pkg) is called
  THEN a new graph must be returned with those units registered.

• GIVEN a UnitPackage with edge definitions
  WHEN graph.with_package(pkg) is called
  THEN the new graph must have conversion edges for those definitions.

• GIVEN a unit defined in a package
  WHEN Number(1, "custom_unit").to("base_unit") is called within using_graph(extended_graph)
  THEN the conversion must succeed using the package-defined edge.

• GIVEN multiple packages
  WHEN graph.with_package(a).with_package(b) is called
  THEN units and edges from both packages must be available.

• GIVEN a package with an invalid dimension name
  WHEN load_package(path) is called
  THEN PackageLoadError must be raised with a descriptive message.

• GIVEN a package file that does not exist
  WHEN load_package(path) is called
  THEN PackageLoadError must be raised.

• GIVEN a package file with invalid TOML syntax
  WHEN load_package(path) is called
  THEN PackageLoadError must be raised with parse error details.

• GIVEN graph.with_package(pkg)
  WHEN the original graph is inspected
  THEN it must be unchanged (immutable composition).

• GIVEN an extended graph
  WHEN set_default_graph(extended_graph) is called
  THEN subsequent conversions must use the extended graph by default.