polish(sprint-0): clean code per ruff + mypy strict

Run the dev linters over Sprint 0's core/library/shared modules and
address every finding. Behaviour unchanged; tests still 80/80 green.

Changes:

- Replace `class Foo(str, Enum)` with `class Foo(StrEnum)` (PEP 663
  / Python 3.11+) in 7 enum classes: ActuatorType, AlarmSeverity,
  AlarmType, KnobMode, KnobFunction, AutopilotMode, AccessLevel,
  VesselType. Pydantic v2 serialises StrEnum the same way, so YAML/JSON
  round-trips are byte-identical.
- Use `datetime.UTC` alias in place of `datetime.timezone.utc`
  (UP017) across alarms.py, knob_state.py, project_config.py, and
  test_knob_state.py.
- Remove now-unnecessary forward-reference quotes from method return
  type annotations (UP037) — `from __future__ import annotations` is
  already in scope everywhere.
- Tighten `_read_json_resource` / `_read_yaml_resource` in the library
  loader: validate that the deserialised payload is actually a dict
  before returning, instead of leaking `Any` from json.loads /
  yaml.safe_load. Fixes the only two `mypy --strict` findings.
- Add `.claude/settings.local.json` to .gitignore (personal
  Claude Code overrides are not committed).

Verification:
  ruff check arautopilot/                 -> All checks passed
  mypy arautopilot/core library shared    -> Success, 0 issues, 12 files
  pytest                                  -> 80 passed in 0.25s

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

arautopilot/core/vessel_config.py

@@ -6,7 +6,7 @@ of every ``ProjectConfig``.
 
 from __future__ import annotations
 
-from enum import Enum
+from enum import StrEnum
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -15,7 +15,7 @@ from arautopilot.core.ids import VesselId, new_vessel_id
 from arautopilot.core.pid_config import PidConfig
 
 
-class VesselType(str, Enum):
+class VesselType(StrEnum):
     """Vessel classes targeted by Phase 1 of the product (brief section 3)."""
 
     YACHT_MOTOR_PLANEO = "yacht_motor_planeo"