fix: Use immutable copy pattern for WellItem result attachment (#1166)

## 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:
```python
# 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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.1 <noreply@anthropic.com>

Author: nathan-stender

src/allotropy/parsers/appbio_quantstudio/appbio_quantstudio_data_creator.py

@@ -241,8 +241,6 @@ def _create_measurement(
 ) -> Measurement | None:
     if not result:
         return None
-    # TODO: temp workaround for cal doc result
-    well_item._result = result
 
     (
         reporter_dye_data_cube,
@@ -332,6 +330,30 @@ def get_well_item_results(
     )
 
 
+def enrich_wells_with_results(
+    wells: list[Well],
+    results_data: dict[int, dict[str, Result]],
+) -> list[Well]:
+    """Return new Wells with results attached to well items via immutable copy.
+
+    This creates new WellItem instances with _result properly set at construction time
+    (via dataclasses.replace), avoiding mutation of the private field after creation.
+    """
+    from dataclasses import replace
+
+    enriched_wells = []
+    for well in wells:
+        enriched_items = []
+        for well_item in well.items:
+            result = get_well_item_results(well_item, results_data)
+            if result:
+                enriched_items.append(replace(well_item, _result=result))
+            else:
+                enriched_items.append(well_item)
+        enriched_wells.append(replace(well, items=enriched_items))
+    return enriched_wells
+
+
 def get_well_item_amp_data(
     well_item: WellItem, amp_data: dict[int, dict[str, AmplificationData]]
 ) -> AmplificationData | None:

src/allotropy/parsers/appbio_quantstudio/appbio_quantstudio_parser.py

@@ -8,6 +8,7 @@
     create_calculated_data,
     create_measurement_groups,
     create_metadata,
+    enrich_wells_with_results,
 )
 from allotropy.parsers.appbio_quantstudio.appbio_quantstudio_reader import (
     AppBioQuantStudioReader,
@@ -41,8 +42,11 @@ def parse_data(
         results_data, result_metadata = Result.create(reader, header.experiment_type)
         melt_data = MeltCurveRawData.create(reader)
 
+        # Create immutable copies of wells with results attached
+        enriched_wells = enrich_wells_with_results(wells, results_data)
+
         calculated_data_documents = iter_calculated_data_documents(
-            [well_item for well in wells for well_item in well.items],
+            [well_item for well in enriched_wells for well_item in well.items],
             header.experiment_type,
             result_metadata.reference_sample_description,
             result_metadata.reference_dna_description,
@@ -52,7 +56,7 @@ def parse_data(
             metadata=create_metadata(header, original_file_path),
             measurement_groups=create_measurement_groups(
                 header,
-                wells,
+                enriched_wells,
                 amp_data,
                 multi_data,
                 results_data,