Add unit-aware Param() field and with_units()/to_example() to meshly

[afshawnlotfi]

Summary

Adds Param(), a drop-in replacement for pydantic.Field() that carries units, shape, and example metadata through to the JSON schema. Also adds two new methods on Packable: to_example() (build an instance from example/default values) and with_units() (clone with numeric fields converted to pint.Quantity objects). This enables downstream packages like hypersim to declare physical units on simulation parameters and have them appear in tool schemas, MCP integrations, and UI forms without any manual filtering or schema post-processing.

Usage Example

from meshly import Packable, Param, InlineArray

class Simulation(Packable):
    velocity: InlineArray = Param(units="m/s", example=[30.0, 0, 0], shape=(3,))
    temperature: float = Param(300.0, units="K", description="Fluid temperature")
    pressure: float = Param(101325.0, units="Pa")

# Units in JSON schema
schema = Simulation.model_json_schema()
# schema["properties"]["temperature"]["units"] == "K"

# Build from examples
sim = Simulation.to_example()
# sim.velocity == np.array([30., 0., 0.])

# Convert to pint Quantities
sim_units = sim.with_units()
# sim_units.velocity == Quantity([30. 0. 0.], 'm/s')
# sim_units.velocity.to("km/h") == Quantity([108. 0. 0.], 'km/h')