feat(perf): large project scaling tests (#984)

## Summary

Add large project scaling benchmarks and tests at production scale (10K–100K files).

### New ASV Benchmarks

**IndexingScalingSuite** (`large_project_scaling_bench.py`):
- `time_walk_and_index` at 1K/10K/50K/100K files
- `time_incremental_refresh` (1% modified files)
- `track_indexed_file_count`, `track_tokens_per_second`

**ContextAssemblyScalingSuite** (`context_assembly_scaling_bench.py`):
- `time_full_pipeline` at 100/1K/5K/10K fragments
- `time_tiered_strategy`, `time_recency_strategy`
- `track_assembled_tokens`, `track_fragments_per_second`

**ExecutionThroughputSuite** (`execution_throughput_bench.py`):
- `time_sequential_plans` at 10/50/100 plans
- `time_executor_construction`, `time_decision_tree_scaling`

### Scale Fixture Updates

- Added `xlarge` (50K files) and `xxlarge` (100K files) profiles to `scale_metadata.json`
- Added 50K/100K thresholds to `baseline_thresholds.json`
- Added `context_assembly` and `execution_throughput` threshold sections

### Tests & Documentation

- 15 Behave scenarios validating profiles, thresholds, monotonicity, memory budgets
- 6 Robot integration tests including live 1K-file indexing throughput check
- `docs/reference/scaling_baselines.md` documenting all baseline metrics

### Quality Gates

| Session | Result |
|---|---|
| `nox -s lint` | PASS |
| `nox -s typecheck` | PASS (0 errors) |
| `nox -s unit_tests` | PASS (10,910 scenarios) |
| `nox -s integration_tests` | PASS (1,526 tests) |
| `nox -s coverage_report` | 97% (>= 97%) |

Closes #859

Reviewed-on: #984
Co-authored-by: Brent E. Edwards <brent.edwards@cleverthis.com>
Co-committed-by: Brent E. Edwards <brent.edwards@cleverthis.com>

benchmarks/plan_explain_bench.py

@@ -92,7 +92,6 @@ class PlanExplainSuite:
             self.decision,
             show_context=True,
             show_reasoning=True,
-            show_alternatives=True,
         )