Graph-local name resolution

[withtwoemms]

As a library user extending ucon with custom units
I want unit name resolution to be scoped to the active ConversionGraph
So I can define units that are visible only within a specific graph context without polluting global state

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Acceptance Criteria

• GIVEN a custom Unit registered via graph.register_unit(unit)
  WHEN get_unit_by_name(name) is called within using_graph(graph)
  THEN the custom unit must be returned.

• GIVEN a custom Unit registered on graph A
  WHEN get_unit_by_name(name) is called within using_graph(graph_b) (a different graph)
  THEN the custom unit must NOT be found (raises UnknownUnitError or returns None).

• GIVEN a unit name that exists in both graph-local and global registries
  WHEN get_unit_by_name(name) is called within using_graph(graph)
  THEN the graph-local unit must take precedence.

• GIVEN a unit name that exists only in the global registry
  WHEN get_unit_by_name(name) is called within using_graph(graph)
  THEN the global unit must be returned (fallback behavior).

• GIVEN a ConversionGraph
  WHEN graph.copy() is called
  THEN a new independent graph must be returned with copied name registries.

• GIVEN the default graph built by _build_standard_graph()
  WHEN any standard unit name is queried via graph.resolve_unit(name)
  THEN the standard unit must be returned.

• GIVEN case-sensitive unit shorthands (e.g., 'm' for meter, 'M' for mega prefix)
  WHEN graph.resolve_unit(name) is called
  THEN case-sensitive lookup must be attempted before case-insensitive fallback.

• GIVEN nested using_graph() contexts
  WHEN the inner context exits
  THEN the outer graph must be restored for both conversion and name resolution.