fix: prevent Task.run from propagating dependency-resolution failures

[oliverm91]

Summary

Task.run resolved dependency arguments (_resolve_args) outside the
retry try/except. When injecting an upstream result failed, the exception
propagated out of Task.run — which crashed the whole sequential run,
while parallel execution silently swallowed it. This PR removes that asymmetry
and tightens the TaskResult construction surface along the way.

What changed

Fix — Task.run never propagates (51f4192)

• Arg resolution now runs inside the error handling, so any failure
  (before a single attempt) is returned as an ERRORED TaskResult
  (attempts=0), exactly like a failure inside the task's own function.
• Factored the failure path (log + ErrorData + wrap) into _errored_result,
  shared by the resolution-failure path and the retry-exhausted path.
• Added TaskResult named constructors — pending(), skipped(),
  success(...), errored(...) — used by the new error path.

Refactor — cleaner state handling (99704d7)

• Split the cascade-skip query from its mutation: _has_failed_dep is
  now a pure predicate; callers record SKIPPED explicitly. (Previously
  _is_unrunnable mutated results as a side effect of a boolean check.)
• Adopted the named constructors across ProcessRunner.
• Use the existing TaskResult.worked property instead of re-deriving
  status == SUCCESS by hand in the runner and the report builder.

Behavior change

A dependency-resolution failure used to raise out of Process.run(parallel=False).
It is now reported as an ERRORED task in the ProcessExecutionReport, and the
rest of the (independent) graph continues — matching how every other task
failure is already handled, and matching parallel execution.

Tests

New tests/test_resolve_args_failure.py:

• test_run_wraps_resolution_failure — unit: a resolution failure yields
  ERRORED with attempts=0, no raise.
• test_sequential_process_survives_resolution_failure — end-to-end: sequential
  Process.run no longer crashes (this fails against the previous code).

Type of change

• feat — new feature
• fix — bug fix
• refactor — no behavior change
• docs — documentation only
• test — tests only
• chore / ci / build