fix: Handle QuantStudio Stop Time and non-ASCII delta columns (#1203)

## Summary
- Accept "Experiment Run Stop Time" as alias for "Experiment Run End
Time" in header metadata parsing (some QuantStudio exports use this
alternate field name)
- Explicitly read `ΔΔCт - Fsigma` and `ΔΔCт + Fsigma` columns with valid
Python identifiers before `get_unread()`, preventing `TypeError: Field
names must be valid identifiers` when these flow through
`make_dataclass()`

## Test plan
- [x] Added `appbio_quantstudio_example10.xlsx` test file exercising
both fixes (384-well, Comparative Ct with "Stop Time" and ΔΔCт columns)
- [x] All 28 QuantStudio parser tests pass
- [x] All 312 vendor discovery tests pass
- [x] Lint clean

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

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: nathan-stender

src/allotropy/parsers/appbio_quantstudio/appbio_quantstudio_structure.py

@@ -82,7 +82,9 @@ def create(data: SeriesData) -> Header:
                 plate_well_count = int(plate_well_count_search.group())
 
         return Header(
-            measurement_time=data[str, "Experiment Run End Time"],
+            measurement_time=data[
+                str, ("Experiment Run End Time", "Experiment Run Stop Time")
+            ],
             plate_well_count=plate_well_count,
             experiment_type=assert_not_none(
                 experiments_type_options.get(
@@ -474,6 +476,8 @@ def create_result(
                     "expfail": data.get(str, "EXPFAIL"),
                     "tholdfail": data.get(str, "THOLDFAIL"),
                     "prfdrop": data.get(str, "PRFDROP"),
+                    "delta_delta_ct_minus_fsigma": data.get(float, "ΔΔCт - Fsigma"),
+                    "delta_delta_ct_plus_fsigma": data.get(float, "ΔΔCт + Fsigma"),
                     **data.get_unread(
                         skip={
                             "Well",