chore: version-control sweep analysis scripts

Copy analyze_sweep.py, check_noise.py, compare_with_grid.py and
compare_repro.py out of the git-ignored results/ tree into scripts/sweep/.
Replace the implicit script-location repository root with a --repo-root
argument (relative default) without changing any computation.  Outputs were
verified byte-identical to the originals on a scratch copy of the data.

scripts/sweep/README.md

@@ -0,0 +1,99 @@
+# Sweep analysis scripts
+
+Read-only analysis helpers for the completed seed-0 sweeps. They were originally
+written inside `results/sweep/` and `results/sweep_repro/` (both git-ignored);
+these copies are the version-controlled source.
+
+All four scripts accept `--repo-root` (default: the repository that contains
+`scripts/sweep/`, i.e. two levels up from the script). Data is read from the
+sweep run directories and, where noted, from the earlier grid runs and
+`artifacts/analysis/`. Nothing is retrained.
+
+## `analyze_sweep.py`
+
+Inputs
+- `results/sweep/runs/<dataset>__alpha<alpha>__seed0__support_weighted__beta<beta>/COMPLETE`
+- `.../results/<run>/val_selection.json`
+- `.../results/<run>/test_global_metrics.json`
+- `results/<run>/val_selection.json` and `test_global_metrics.json` for the earlier
+  grid runs (stage 3)
+
+Outputs
+- `results/sweep/analysis/selection.csv` (validation only)
+- `results/sweep/analysis/test_at_selected_beta.csv`
+- `results/sweep/analysis/repro_vs_previous.csv`
+
+Command
+```bash
+python3 scripts/sweep/analyze_sweep.py
+# or, against a different tree:
+python3 scripts/sweep/analyze_sweep.py --repo-root /path/to/repo
+```
+
+## `check_noise.py`
+
+Inputs
+- `results/sweep/runs/<run>/results/<run>/{history.json,val_selection.json,test_global_metrics.json}`
+- `results/<run>/{history.json,val_selection.json,test_global_metrics.json}` (earlier grid)
+
+Outputs
+- `results/sweep/analysis/noise_checks.json`
+
+Command
+```bash
+python3 scripts/sweep/check_noise.py
+# or:
+python3 scripts/sweep/check_noise.py --repo-root /path/to/repo
+```
+
+## `compare_with_grid.py`
+
+Inputs
+- `results/sweep/analysis/selection.csv` (run `analyze_sweep.py` first)
+- `artifacts/analysis/final__six_condition_master.csv`
+- `artifacts/analysis/<dataset>__alpha<alpha>__multiseed__validation_per_seed.csv`
+
+Outputs
+- `results/sweep/analysis/compare_with_grid.csv`
+
+Command
+```bash
+python3 scripts/sweep/compare_with_grid.py
+# or:
+python3 scripts/sweep/compare_with_grid.py --repo-root /path/to/repo
+```
+
+## `compare_repro.py`
+
+Inputs
+- `results/sweep_repro/a_1thread_repeat/results/<run>/`
+- `results/sweep_repro/b_8thread/results/<run>/`
+- `results/sweep/runs/<run>/results/<run>/`
+- `results/<run>/` (earlier grid)
+- prototype snapshots under `results/sweep_repro/*/analysis/`,
+  `results/sweep/runs/<run>/analysis/` and `artifacts/analysis/`
+
+where `<run>` is `xiiotid__alpha0.25__seed0__support_weighted__beta0`.
+
+Outputs
+- `results/sweep_repro/compare_repro.json`
+- a compact comparison table on stdout
+
+Command
+```bash
+python3 scripts/sweep/compare_repro.py
+# or:
+python3 scripts/sweep/compare_repro.py --repo-root /path/to/repo
+```
+
+## Notes
+
+- `results/` is git-ignored. These scripts write only into
+  `results/sweep/analysis/` and `results/sweep_repro/`; they never write into a
+  tracked file.
+- `analyze_sweep.py` opens validation data in stage 1 and only opens test data
+  afterwards, by construction. The scripts do not select or recommend a beta for
+  the thesis; they print descriptive tables.
+- The computation is unchanged from the originals. The only change is that the
+  repository root is a `--repo-root` argument with a relative default instead of
+  being derived from the script's old location inside `results/`.