v0.9.0 — the gate finishes, and says what it measured

The release that makes ditto's own gate finish, by making it ask a question it
can afford.

Added

• ditto changed --since <ref>, and PlanChanged / RunChanged behind it.
  It is staged asked of a committed range instead of the index: the scope is
  <ref>...HEAD, the diff against their merge base, so a base that has moved on
  does not drag somebody else's commits into the bill. The diff parsing, the byte
  offsets, the fail-open rule and the sandbox are the staged path's, unchanged.

  It exists because ditto staged cannot be a CI gate. A CI checkout has nothing
  staged — the change is already committed — so a gate pointed at the staged
  scope skips, reports success, and measures nothing.

  It refuses a checkout with staged changes in it. A range scope names bytes of
  HEAD while the sandbox is written from the index, and those agree exactly
  while the index agrees with HEAD; scoping against one and mutating the other is
  the defect already measured at seven of eight verdicts moving. A worktree-only
  modification never reaches the sandbox and is allowed — written the other way
  round first, it refused ditto's own CI in fifty-one seconds, because the Devbox
  install step modifies a tracked lockfile.

  There is no default base, and there will not be one: on a CI checkout the
  useful base is the last release, on a branch it is the trunk, and a base
  guessed wrong is either a bill nobody asked for or a scope of nothing reported
  as green.

Changed

• ditto's own CI gate mutates the change rather than the repository. The
  repository-sized run asks for 783 mutants and dies at its thirty minutes having
  reached about 424, measured four times. Both levers were already spent: gating
  removes 54% of the compilations and does not close it, and cutting the mutant's
  suite by 46% moved the gate by 0.5%, because -failfast already stops a killed
  mutant at its first failing test. The bill was the wrong size rather than badly
  paid — backlog entry 21, open since the measurement and now closed.

  make test.mutation is untouched and workflow_dispatch still reaches it. The
  repository-sized question is worth asking on purpose; it was being asked on
  every push, against a clock it could not beat.

Measured

This repository's own gate, before and after:

Run	Result
Repository-sized, on the 0.8.0 merge	FAIL … 1800.181s — the -timeout=30m exactly
Changed gate, on this release's merge	PASS in 153.96s — 49 mutants, score 0.90 against a 0.50 minimum