Guard the export progress divisions that produced Infinity

[jcschaff]

Root-cause companion to #1837, which contains the symptom. This stops the bad value being produced.

Cause

Export progress denominators are counts that can legitimately be zero, and they are doubles — so x / 0.0 silently yields Infinity (or NaN when x is 0) instead of throwing. That value was published as export progress, reached JSON serialization, and made the event permanently undeliverable — ~4,900 error lines a minute on production until the pods were rolled.

The reachable case is ASCIIExporter's TOTAL_EXPORTS_OPS:

SIM_COUNT * PARAMSCAN_COUNT * variableNames.length
  * (geometrySpecs.getCurves().length + (getPointCount() > 0 ? 1 : 0))

zero for a geometry selection with no curves and no points, or with no variables chosen. RasterExporter.fireThrottledProgress divides by endIndex+1, zero for an empty range.

Correction to my earlier analysis in #1837: I first suspected ASCIIExporter's per-variable loop. That is not reachable — both enclosing loops guarantee variableNames.length >= 1 and endIndex >= beginIndex, so that denominator is always ≥ 2. The real culprit is the block above.

Changes

• ASCIIExporter.fraction(completed, total) — returns a value in [0,1], and 0 when there is nothing to divide by. The four progress divisions use it, and a zero total is logged once with the counts that caused it, so the offending export is identifiable.
• RasterExporter.fireThrottledProgress — same helper.
• ClientExportEventController.fireExportProgress — the single place every EXPORT_PROGRESS event is constructed, now rejects a non-finite value as a backstop. Guarding ~15 scattered call sites individually would be easy to get wrong; this one chokepoint is provably complete.

Verified

fraction(  5.0, 10.0) = 0.5   finite, in [0,1] ✓
fraction(  0.0,  0.0) = 0.0   ✓   (was NaN — no curves and no points)
fraction(  3.0,  0.0) = 0.0   ✓   (was Infinity)
fraction( -1.0, 10.0) = 0.0   ✓
fraction( 15.0, 10.0) = 1.0   ✓
fraction(  NaN, 10.0) = 0.0   ✓

Both edited exporters are CRLF files; line endings are preserved (diff is 45 insertions / 5 deletions, no whitespace churn).

🤖 Generated with Claude Code

https://claude.ai/code/session_01SY1XHgTZXZPqUECo2VBAWg