viz/interactive.py: dedup repeated status Div, log-transform closure, and 3D mesh cache key (+ first unit tests)

[sbryngelson]

toolchain/mfc/viz/interactive.py repeats three small, self-contained pieces of logic that are pure functions and trivially extractable. Dedup'ing them removes a live copy-paste hazard (change one copy, forget the others) and is a good opportunity to add the first automated tests for this module.

This is the low-risk subset carved out of the larger _update decomposition (tracked separately) — it touches no callback control flow and is unit-testable.

1. Status html.Div built 4× verbatim

Lines 1971, 2120, 2470, 2626 are the same 8-child html.Div (step {step} / · shape {raw.shape} / Br / min+value / max+value, same _YELLOW/_MUTED/_BLUE/_RED styles), differing only in the min/max local names.
→ add _status_div(step, shape, dmin, dmax) -> html.Div and call it at all four sites.

2. Log/identity transform closure _tf redefined 4×

Lines 379, 386 (inside _prefetch_3d_mesh) and 1853, 1861 (inside _update):

def _tf(arr):
    return np.where(arr > 0, np.log10(np.maximum(arr, 1e-300)), np.nan)
# else
def _tf(arr):
    return arr

→ add _log_transform(log: bool) returning the transform (and the cmin/cmax pair, since sites 379/386 inline-compute those right next to the closure — extract them together so the cmin/cmax duplication doesn't get left behind). The 1e-300 floor should become a module constant _LOG_FLOOR at the same time.

3. 11-arg 3D mesh cache-key tuple repeated

The literal (step, var, mode, log, vmin_in, vmax_in, iso_min_frac, iso_max_frac, iso_n, vol_min_frac, vol_max_frac) appears at 350 (_prefetch_3d_mesh) and 432 (_get_cached_3d_mesh), and is passed positionally from _update at ~1890.
→ add _mesh3_key(...) -> tuple (or a small namedtuple) and use it at all three sites.

Tests

toolchain/mfc/viz/test_viz.py currently has no coverage of interactive.py at all. Add a few unit tests for _status_div, _log_transform, and _mesh3_key — these would be the first tests for this module and make the dedup verifiable.

Behavior preservation / risk: all three are pure-function extractions with no control-flow change; toolchain-only, no golden-file impact.

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_{Filed from a repo-wide code-cleanliness review; verified against master @ 40dde5e.}

Code references

• toolchain/mfc/viz/interactive.py:1971 — status Div copy 1 of 4 (others: 2120, 2470, 2626)
• toolchain/mfc/viz/interactive.py:379-386 — _tf closures (also 1853, 1861)
• toolchain/mfc/viz/interactive.py:350 — 11-arg cache key (also 432, ~1890)
• toolchain/mfc/viz/test_viz.py — test file (no interactive.py coverage yet)