feat: add --slice flag for worker scenario grouping

[cidrblock]

Summary

Adds a --slice flag (and config file support) to control how scenarios are grouped into work units when using --workers. With --slice=1 (the default), scenarios are grouped by their top-level directory segment so all CRUD states for a resource run sequentially on one worker. With --slice=2, each leaf scenario is dispatched independently (pre-slice behavior).

This is relevant for collections with nested scenario directories (e.g. appliance_vlans/gathered, appliance_vlans/merged) where related scenarios share state and must execute in order within a worker.

Changes by file

src/molecule/click_cfg.py — New slice property on CliOptions (experimental, default "1").

src/molecule/types.py — slice: int added to both CommandArgs and ConfigData TypedDicts.

src/molecule/command/test.py, check.py, destroy.py — "slice" added to @common_options and wired into command_args via int(ctx.params["slice"]). No other runtime changes.

src/molecule/config.py — slice property reads from the config dict (self.config.get("slice", 1)), following the same pattern as shared_state. _apply_cli_overrides extended so CLI --slice takes precedence over the config file value.

src/molecule/data/molecule.json — Schema entry for slice (integer, default 1, minimum 1).

src/molecule/scenarios.py — slice property on Scenarios reads from the first scenario's config, mirroring the shared_state aggregation pattern.

src/molecule/worker.py — Core implementation:

• _slice_key(name, depth) — splits scenario name on / and joins the first depth segments to derive the grouping key.
• _group_scenarios_by_slice(scenarios, depth) — groups Scenario objects by _slice_key, preserving original sort order.
• run_scenario_slice(entries, ...) — executes a list of (molecule_file, name) pairs sequentially in one worker process, stopping on first failure within the slice. Returns list[SliceResult].
• run_scenarios_parallel — refactored to group via _group_scenarios_by_slice and submit one future per group to run_scenario_slice instead of one future per scenario. Fail-fast and continue-on-failure semantics apply at the slice level. Log output reports slice count and depth.
• validate_worker_args — validates that --slice (non-default) requires --workers > 1.
• run_one_scenario remains unchanged and exported for backward compatibility but is no longer called internally — run_scenario_slice handles all dispatch.

pyproject.toml — verbosity_assertions changed from 2 (int) to "2" (string). Newer pytest requires this config option to be a string; the int type caused TypeError in all integration tests with assertion failures. Pre-existing issue, not introduced by this PR.

tests/fixtures/integration/test_workers/ — Fixture scenarios restructured from flat (scenario_0..5) to nested (group_a/scenario_0..2, group_b/scenario_3..5) to enable slice testing. config.yml now includes slice: 1.

tests/unit/test_worker.py — New tests for _slice_key (4 cases), _group_scenarios_by_slice (4 cases), run_scenario_slice (success + failure-stops-early), validate_worker_args slice validation (2 cases). Existing run_scenarios_parallel tests updated for the new list[SliceResult] return format.

tests/unit/test_click_cfg.py — test_slice_option verifying the new CLI option properties.

tests/integration/test_workers.py — Scenario references updated for nested names. Two new tests: test_workers_slice_groups_by_resource (verifies 2 slices at depth=1) and test_workers_slice_depth_2 (verifies 6 slices at depth=2).

docs/guides/parallel.md — Documents --slice with a behavior table, CLI examples, config file example, and precedence notes.

Runtime behavior

Existing --workers behavior is preserved. The default --slice=1 groups scenarios by their first path segment, which is a change from pre-slice behavior (individual dispatch) but produces the same results since scenarios within a group still execute sequentially. Users can opt into the previous individual-dispatch behavior with --slice=2.

Sequential mode (--workers 1 or no --workers) is completely unaffected — slice in the config is silently ignored.

Test plan

• Unit tests pass (_slice_key, _group_scenarios_by_slice, run_scenario_slice, validation, parallel dispatch)
• Integration tests pass (slice depth 1 and 2, parallel success, verbose, continue-on-failure, fail-fast)
• tox -e lint clean (ruff, cspell, pydoclint, pylint, mypy, codespell)

[ssbarnea]

@cidrblock bring it out of draft once is passing all CI checks.