ci: impute a weight for new PostgreSQL tests instead of refusing them

[jason931225]

The defect

#815 packs the PostgreSQL shards by measured duration and fails closed on any workflow entry with no measured_seconds. But weights are harvested from cargo-postgres-timing: lines in a finished CI log — so a test that has never run cannot have a measurement, and it cannot run until its PR is green.

Every new PostgreSQL test is currently unmergeable. #802 is the first to hit it:

1 workflow entr(ies) have no measured_seconds; regenerate weights before trusting the balance:
  ontology-canonical-adapter-postgres-employment-reassign-as-runtime-role-pg

Why the guard was still right, and what changes

That guard was written for a different failure — a map that has drifted, where entries silently lost weights they once had. Both look identical to a per-entry check. They differ by scale: a PR adds one or two, a bad regeneration drops dozens. So the distinction is now made on share.

case	before	after
one new test	❌ blocked	imputed + named, exit 0
>10% unmeasured (drift)	❌ blocked	❌ still blocked
no measurement anywhere	❌ blocked	❌ blocked, distinct message

• Unmeasured entries are imputed at the mean of their own package where it has a measured sibling (a new suite usually resembles the suite it joins), global mean otherwise.
• Never 0. A free entry lands in the lightest bin — exactly where a heavy newcomer hurts most — and treating unmeasured as free is how the entry-count scheme drifted in the first place.
• Imputation bases are computed over measured entries only, so an imputed value never feeds back into the mean used to weigh the next one.
• The CLI names every imputed entry and the seconds it added, so an estimate stays visible and gets replaced at the next harvest rather than hardening into a fake measurement.

Measured on #802's exact case

imputed 35.4s across 1 unmeasured entry (replace at the next timings harvest):
  ontology-canonical-adapter-postgres-employment-reassign-as-runtime-role-pg
partition ok (5 shards, max 668.3s, spread 1.00x)

Mutation-proven on the real 209-entry map

mutation	result
strip 15% of measurements	exit 1 — "14.8%, limit 10%; the map has drifted"
strip every measurement	exit 1 — "no basis to impute from"
one new entry (#802's shape)	exit 0 — imputed and named

26 partition tests pass; 12 gates swept, 0 failed; cargo-map, dark-suites and executed-tests-baseline all exit 0.

Unblocks

#802, and every future PostgreSQL test.

🤖 Generated with Claude Code