feat(workflow): add checkpointed rolling generate resumable workflow

[fcogidi]

Summary

• Split the generate workflow into focused internal modules for setup, planning, preparation, and execution.
• Keep results.jsonl as the live artifact while moving checkpointing to SQLite with strict resume validation.
• Add planner-backed resume handling, clearer shutdown behavior, and targeted readability comments for the new workflow seams.
• Update CLI/docs and tests to match the new workflow structure and interrupt-safe cleanup path.

Testing

• uv run pytest -q
• UV_CACHE_DIR=/tmp/uv-cache uv run pre-commit run -a

[Copilot]

Pull request overview

This PR refactors infermesh generate into a checkpointed, rolling-window workflow that can resume interrupted runs using a SQLite checkpoint DB (with strict validation), while keeping results.jsonl as the live user-facing artifact.

Changes:

• Introduces a modular generate workflow engine (_workflow/*) with rolling scheduling, per-row persistence, and strict resume planning/validation.
• Updates CLI to use the new workflow, adds --checkpoint-dir / INFERMESH_CHECKPOINT_DIR, and improves error surfacing.
• Adds comprehensive tests plus docs/README updates for the new resume + mapper behavior.

Reviewed changes

Copilot reviewed 20 out of 20 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/test_workflow.py	New end-to-end unit tests for rolling execution, checkpointing, and strict resume validation.
tests/test_sync_runner.py	Adds a regression test ensuring SyncRunner cancels and allows cleanup on KeyboardInterrupt.
tests/test_cli.py	Updates CLI tests for new resume/checkpoint behavior, mapper support, and error handling.
tests/fakes.py	Extends fakes to support async generation, checkpoint DB helpers, and resume assertions.
src/infermesh/sync_runner.py	Reworks run() to cancel loop-owned tasks on KeyboardInterrupt and allow coroutine cleanup.
src/infermesh/client.py	Routes sync wrappers through the new _run_sync() helper.
src/infermesh/cli.py	Switches generate to run_generate_workflow, adds checkpoint-dir plumbing, and uses managed client lifecycle.
src/infermesh/_workflow/source.py	Adds source parsing, fingerprinting, stdin materialization, and input/output path validation helpers.
src/infermesh/_workflow/runtime.py	Centralizes generate-run setup/cleanup (staging, persistence sink, resume plan, preparer selection).
src/infermesh/_workflow/resume.py	Implements strict resume validation and a SQLite-backed planner for file-backed resumes.
src/infermesh/_workflow/prepare.py	Adds preparers for sequential runs and planner-driven resumes; maps rows into work items or immediate error rows.
src/infermesh/_workflow/models.py	Introduces internal workflow dataclasses (checkpoint keys, work items, resume plan, etc.).
src/infermesh/_workflow/mapping.py	Adds mapper loading, built-in mapping behavior, metadata validation, and mapping fingerprinting.
src/infermesh/_workflow/engine.py	Implements rolling-window scheduler, per-item emission/persistence, and the public workflow entrypoint.
src/infermesh/_workflow/checkpoint.py	Adds SQLite checkpoint schema, staging/bootstrap logic, and a single-threaded persistence sink.
src/infermesh/_workflow/init.py	Exposes run_generate_workflow from the internal workflow package.
src/infermesh/_client_runtime.py	Adds _run_sync() helper on the client runtime to centralize sync execution via SyncRunner.
src/infermesh/_cli_support.py	Adds missing deployments table validation and re-homes _build_generation_record for reuse by the workflow engine.
docs/guide.md	Updates resume docs to describe SQLite checkpointing, strict validation, rolling window behavior, and --mapper.
README.md	Updates generate/resume examples to reference checkpoint files and adds a mapper example.

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