fix: Use immutable copy pattern for WellItem result attachment

[nathan-stender]

Summary

Resolves P0 TODO #2 from codebase audit by using dataclasses.replace() for immutable result attachment instead of mutating private _result field after construction.

Problem

Previously, WellItem._result was mutated inside _create_measurement() with a TODO comment:

# TODO: temp workaround for cal doc result
well_item._result = result

This violated the intended immutability of the private field. The TODO was added during PR #546 refactor when Result changed from a public mutable field to a private field with property accessor.

Root cause: WellItem and Result data come from different file sections parsed at different times, but calculated documents need access to result data through well items.

Solution

Use dataclasses.replace() to create immutable copies with results attached at the correct point in the data flow:

1. Parse wells (from "Sample Setup" section)
2. Parse results (from "Results" section)
3. Create enriched copies via enrich_wells_with_results() using dataclasses.replace()
4. Use enriched wells for calculated documents and measurements

Changes

• Added enrich_wells_with_results() that returns new Well/WellItem instances with results attached
• Removed TODO and mutation from _create_measurement()
• Parser now uses enriched_wells for both calculated documents and measurements
• Clear architectural separation: "wells without results" → "wells with results"

Testing

• All 27 appbio_quantstudio tests pass
• All 506 tests pass
• No golden file changes
• Linting clean

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