MCP needs to support units extensions

[withtwoemms]

As an AI agent using ucon via MCP
I want to define custom units and conversions at runtime
So I can perform conversions for domain-specific units that are not in ucon's core library

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Acceptance Criteria

Session Tools

• GIVEN a call to define_unit(name, dimension, aliases)
  WHEN the dimension is valid
  THEN the unit must be registered in the session graph and a confirmation returned.

• GIVEN a call to define_unit(name, dimension, aliases)
  WHEN the dimension is invalid
  THEN a structured error with suggestions must be returned.

• GIVEN a unit registered via define_unit
  WHEN define_conversion(src, dst, factor) is called with that unit
  THEN a conversion edge must be added to the session graph.

• GIVEN units and edges registered via session tools
  WHEN convert(value, from_unit, to_unit) is called
  THEN the conversion must succeed using session-defined units.

• GIVEN a session with custom definitions
  WHEN reset_session() is called
  THEN all custom units and edges must be cleared.

• GIVEN a reset session
  WHEN a previously-defined custom unit is used in convert()
  THEN an unknown_unit error must be returned.

Inline Parameters (Recovery Path)

• GIVEN no prior session state
  WHEN convert(..., custom_units=[...], custom_edges=[...]) is called
  THEN the conversion must succeed using the inline definitions.

• GIVEN inline definitions on convert()
  WHEN the call completes
  THEN the session graph must NOT be modified (inline is ephemeral).

• GIVEN inline definitions on compute()
  WHEN the call completes
  THEN multi-step calculations must work with inline-defined units.

• GIVEN an unknown_unit error from a prior convert() call
  WHEN the agent retries with inline custom_units and custom_edges
  THEN the conversion must succeed (recovery pattern).

Caching (Optimization)

• GIVEN identical custom_units and custom_edges on multiple calls
  WHEN the inline graph is compiled
  THEN the cached graph must be reused (no redundant compilation).