fix(interpolator): add missing test for empty-instances guard; clear two stale NEEDS_FIX markers

[Jammy2211]

Summary

Two NEEDS_FIX markers filed 2026-04-10 blame an IndexError in
InstanceInterpolator.__getitem__ when querying a between-samples time (1.5).
Both markers are stale. The underlying library bug was fixed by
PyAutoFit c8511b553 (2026-04-12) — two days after the markers were filed —
which added the empty-instances guard in autofit/interpolator/abstract.py.

Reproduction on clean main (2026-07-22)

All three repos clean and on main, with output/ moved aside so nothing was
served from cache:

Run	Result
autofit_workspace scripts/features/interpolate.py, real, fresh output/	exit 0
same script under PYAUTO_TEST_MODE=1, no output/ at all	exit 0
HowToFit chapter_1_introduction/tutorial_5_results_and_samples.py, real	exit 0

Corrections to the original diagnosis

• It is not a bracketing / off-by-one bug. t == 1.5 interpolation always
  worked — test_interpolator.py already exercises between-samples queries at
  1.5 in four places. The real failure was an empty instances list reaching
  self.instances[0], as the workspace marker's own text said ("value_map
  lookup falls through to empty instances list").
• HowToFit tutorial_5 contains no interpolator code at all. The "likely
  related to InstanceInterpolator" text in that marker was a misattribution.

Residual gap

c8511b553 shipped the guard with no test. That is the only code change
left to make.

Plan (high level)

1. Add the missing regression test for the empty-instances guard in PyAutoFit.
2. Remove the stale NEEDS_FIX marker from autofit_workspace no_run.yaml.
3. Remove the stale NEEDS_FIX marker from HowToFit no_run.yaml.
4. Record here that the fix was c8511b553 and that the HowToFit marker's
   stated cause was never correct.

Plan (detailed)

• PyAutoFit/test_autofit/interpolator/test_interpolator.py — add
  test_empty_instances_raises: construct af.LinearInterpolator([]), query
  interpolator.t == 1.5, assert pytest.raises(IndexError, match="no instances").
  numpy-only, no JAX. Verify with pytest test_autofit/interpolator/.
• autofit_workspace/config/build/no_run.yaml — delete the
  features/interpolate line.
• HowToFit/config/build/no_run.yaml — delete the
  chapter_1_introduction/tutorial_5_results_and_samples line.
• No notebook regeneration — config-only change in both workspaces.

Branch: feature/interpolator-stale-needs-fix

Out of scope (to be filed separately)

Two exit-0 correctness smells noticed while reproducing, unrelated to the
IndexError:

• HowToFit tutorial_5: the section printing "only the Gaussian normalization
  and sigma" emits 5 values instead of 2.
• autofit_workspace features/interpolate: the aggregator block prints a
  different t=2 centre than the in-memory block (49.96 vs 59.73 in one run).

[Jammy2211]

Shipped — 2 PRs open

• test(interpolator): cover empty-instances IndexError guard #1412 — adds test_no_instances, the regression test c8511b553 omitted. pytest test_autofit/interpolator/ → 36 passed.
• chore(no_run): un-park tutorial_5_results_and_samples (stale NEEDS_FIX) HowToFit#24 — removes the stale tutorial_5 NEEDS_FIX marker.

Scope correction found during implementation

The autofit_workspace half needed no change — the features/interpolate marker was already removed upstream by autofit_workspace PR #103 (7154e8a, "un-park features/interpolate (IndexError fixed)"). My initial grep hit a stale local main. Branch dropped, no PR for that repo.

So of the three repos in the original prompt, only two were actually in play, and neither needed a behavioural fix — the library bug was fixed 2026-04-12.

Notes

• HowToFit's no_run.yaml now has zero entries. Both PyAutoHands loaders (run.py:53, run_python.py:46) do no_run_data or [], so the resulting None parse is handled — no build regression.
• Two unrelated exit-0 correctness smells noticed while reproducing are recorded in the issue body under "Out of scope"; not filed as prompts yet.

[Jammy2211]

CLOSED — all 4 PRs merged

PR	Change
#1412	test_no_instances regression test
PyAutoLabs/HowToFit#24	Un-park tutorial_5 NEEDS_FIX
PyAutoLabs/HowToFit#25	Stale filtering prose
PyAutoLabs/autofit_workspace#104	Select instances by time, not position

All green on Python 3.12 + 3.13 before merge.

Net result: the IndexError this issue was filed for did not exist — fixed by c8511b553 on 2026-04-12, two days after the markers were parked. What the task actually produced was the missing regression test for that fix, removal of two stale NEEDS_FIX markers, and two genuine tutorial-correctness bugs found while reproducing.