The homepage strip floored percentiles into 1-99 while the Day page rendered
"100th pct" for the same reading. Rather than teach the Day page a second copy
of the rule, put it in grading.pct_ordinal() and have every surface defer to it:
the Day page, the calendar tooltip, the chart labels, the city pages and the
strip.
Two bugs fell out of unifying them:
- The city pages rendered `{{ c.percentile }}th pct` against a float, so all
~1000 of them showed "97.1th pct", "66.4th pct", "16.5th pct" — and would say
"1th"/"2th"/"3th" for the low tail. Live in prod.
- Python's round() is half-to-even and JavaScript's Math.round is half-up, so
16.5 gave "16th" server-side and "17th" client-side. The Python helper uses
floor(x + 0.5) to match the frontend exactly; a cross-language check over every
.5 boundary now agrees on all of them.
The frontend mirrors the rule as pctOrd() in shared.js, replacing the bare ord()
at all five percentile call sites (day.js, calendar.js, chart.js x2, app.js).
ord() stays as the general ordinal helper it always was.
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