server: retire hard-timeout ledger

[yisding]

Problem

After every server cleanup caller established explicit ownership, server/transports.py still contained the grandfathered _await_with_hard_timeout helper and _BACKGROUND_TIMEOUT_TASKS module-global ledger. The helper could create hidden work and kept survivors outside their lifecycle owners.

Changes

• add wait_for_owned_future, a shared hard-bound primitive that never creates or retains work
• migrate all server transport, VoiceServer, and WebRTC route deadline waits to caller-owned futures
• preserve timeout cancellation requests and external caller-cancellation behavior
• delete _await_with_hard_timeout, _track_background_timeout, and _BACKGROUND_TIMEOUT_TASKS
• add primitive tests for cancellation-resistant timeout survivors and externally cancelled waiters
• remove the final server raw task start and the repository's final module-global task ledger
• refresh and classify both structural inventories
• document the no-hidden-ledger ownership rule

Why / impact

Server teardown survivors now stay exclusively with their RuntimeTaskScope, SurvivorRegistry, or explicit handler owner. There is no cross-runtime module-global anchor and no timeout helper that can silently turn a coroutine into a task. Repository-wide raw production starts fall from 23 to 22 in this slice (29 to 22 across the post-#930 server sequence), and module_task_set falls from 1 to 0.

Checks

• env UV_FROZEN=1 just check outside the sandbox because uv's cache lock is not sandbox-writable
  • pre-commit: passed
  • mypy: 288 source files passed
  • pytest: 8,591 passed, 309 skipped
• .venv/bin/python -m pytest tests/runtime/test_task_scope.py tests/server/test_capacity_gate_drain.py tests/server/test_websocket_runtime_drain.py tests/server/test_voice_server_lifecycle.py tests/server/test_webrtc_routes.py -q (116 passed, 28 skipped)
• .venv/bin/python -m pytest tests/ratchets/test_teardown_budget_inventory.py tests/ratchets/test_source_inventory.py -q (16 passed)
• focused Ruff and mypy checks
• commit-time pre-commit hooks

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