0.2.0

Added

• Operator plugin API: define custom operators by subclassing
  ActiveMutator::Operators::Base (subclassing is registration), loaded
  via the repeatable --operator FILE flag or the operators: config-file
  key. Relative paths resolve against the project root. See
  docs/guides/custom-operators.md. The Base helpers (edit,
  loc_range) and Edit members are semver-stable from this release.
• Heredoc mutation: plain heredocs with a nonempty (dedented) body get
  an "empty heredoc body" mutant. Interpolated heredocs are untouched.
• class << self mutation: defs inside class << self within a
  constant scope are now discovered as singleton subjects and re-inserted
  through the singleton class. class << obj and top-level class << self
  stay skipped, as do classes/modules declared inside class << self
  (their constants live on the singleton class and are unreachable by
  name).
• Nested def mutation: bodies of defs nested inside another def are
  mutated under the outer subject (a nested def re-executes on every outer
  call, so a separate subject would produce phantom survivors).
• Broader CallSwap pack: all? → any?, take ↔ drop,
  min_by ↔ max_by, sort → reverse, detect/find → first.
• Project config file: .active_mutator.yml at the project root,
  layered under CLI flags (flags win).
• --fail-at SCORE: opt-in gate — exit 0 when the mutation score meets
  the threshold even with survivors.
• Adaptive timeout calibration (on by default, --no-adaptive-timeout
  to disable): grow-only budget scaling (1x–4x) from observed worker wall
  times, so parallel-load slowdown doesn't turn honest kills into false
  timeouts.
• Delta refresh now re-runs non-covering spec files that reference a
  changed file's constants, closing the newly-covering-example blind spot.
• --format stryker-json (mutation-testing-report-schema v2) and
  --format github (PR annotations for survivors).
• Per-operator equivalent-rate metric in run summaries.
• Subject expression language for --subject (Foo::Bar#baz, Foo::Bar,
  Foo::Bar*, Foo::Bar#*).
• --exclude PAT glob filtering (gitignore-like recursive semantics),
  --max-mutants N deterministic sampling, and --debug-plan dry run.
• Per-method opt-out via a # active_mutator:skip comment on the line
  above a def.
• Benchmark harness (bin/bench, bin/bench-diff) with a cross-run
  Stryker report differ.

Fixed

• Positional path arguments: nonexistent paths error out, non-Ruby file
  args are rejected, overlapping path args are deduplicated.
• Scoped --accept-survivors no longer clobbers out-of-scope ledger
  entries; accepted fingerprints referencing missing files warn.
• Covering-example lookup widened to the subject's whole line range.
• Scheduler tolerates non-Hash JSON payloads from workers.
• Unloadable operator files and operators raising during analysis fail
  with attributed, friendly errors instead of raw stack traces.