v1.2.0 — within-stage crash-resume + graceful budget halt

[1.2.0] - 2026-04-21

Within-stage crash-resume. A pipeline killed mid-Stage-N no longer loses
that stage's partial progress — the rerun skips any artifact whose
input hash still matches the checkpoint. Budget-cap halts are now
graceful exits with a resume-message format.

Added

• Within-stage checkpointing. Stages 2 (file summaries), 3 (class
  docs), 4 (package rollups), 5 (mermaid), and 6 (tech-debt topics) now
  write to state.artifact_index after every completed artifact, under
  an asyncio.Lock. Mid-stage crash + rerun never re-calls the LLM for
  already-produced artifacts.
• Atomic artifact writes. New designdoc.io_utils.atomic_write(path, content) writes to a sibling .tmp then os.replace() (POSIX-atomic).
  Every artifact and the .designdoc-state.json file itself use it.
• Graceful budget halt. BudgetExceededError mid-stage now sets
  state.halted_on_budget=True, marks the stage FAILED, saves state, and
  the CLI prints budget exhausted at $X / cap $Y. Run designdoc resume --budget <new-cap> to continue with exit 0.
• Observability. Orchestrator stage-start log lines include the
  count of already-checkpointed artifacts (e.g., [3/9] stage class_docs: 40 artifacts checkpointed).

Changed

• PipelineState.artifact_index is now dict[str, dict[str, str]]
  (was dict[str, str]) carrying {"path": ..., "input_hash": ...}
  per artifact. Old-shape state files are migrated in-memory on load
  with empty input_hash — safe fallback that forces reprocessing.
• Orchestrator.run() no longer re-raises BudgetExceededError. The
  CLI reads state.halted_on_budget after the run and formats the
  resume message itself. Exit code 0 (resumable), not 4 (crash).

Fixed

• Concurrent state.save() under asyncio.gather is now serialized by
  a module-level asyncio.Lock, preventing lost writes when
  parallelism > 1.