2.6.1: the facts 2.6.0 added reach a results store

The facts 2.6.0 added reach a results store.

Shipped as a patch. It changes what published rows contain, so the 1.0.0
policy would call it a minor; it goes out as 2.6.1 as a deliberate exception,
because it completes 2.6.0 rather than adding to it. Same call 2.4.2 made, for
the same reason.

Added

• score_rows exports the direction and range 2.6.0 introduced. That release
  gave every metric both and then kept them in memory: the row shape this
  module flattens a run into carried neither, so nothing reached a results
  store and no consumer downstream could use them. Five columns on every row
  shape, including the failed-invocation row that names no metric at all.
  • value_range (none / bounded / unbounded_above /
    unbounded_below / unbounded), value_minimum and value_maximum.
    The discriminator is what makes "declared unbounded" and "never declared"
    different answers, which two nullable floats cannot express; it is the
    idiom metric_kind = 'none' already uses beside a filled-in value. All
    five members are spelled out because MetricRange can express all five -
    range: {max: 5} on a numeric field produces unbounded_below - and a
    consumer storing this as an enum pays a migration to add a member later.
  • metric_direction and higher_is_better. metric_direction carries the
    same four values gate_win does, none among them, meaning never
    computed: an ungated run has no baseline, and neutral would report
    "measured, and it did not move" about a number nothing was measured
    against. higher_is_better is 'true'|'false'|'unknown', and unknown
    is narrower than it looks - compare() defaults a metric nothing
    mentions to higher-is-better, so it reads unknown only when a suite
    declared the metric neutral or there was no comparison at all.

Changed

• _passed is now _tristate, taking the spelling of its third state as an
  argument. passed keeps 'null', which it has published since the row
  shape existed; higher_is_better uses 'unknown', which reads as "nobody
  declared one" rather than as a missing value. Private; no caller outside
  this module.