Expose the active reactor and support native StreamWriter.start_tls()

[egorsmkv]

Refs #106, specifically the author's additional observations about reactor diagnostics and the missing native StreamWriter.start_tls(). Read cancellation is handled separately in #107.

The build-time reactor label cannot identify automatic io_uring fallback. Native stream writers also lacked the stream-level TLS upgrade API, and the existing low-level upgrade did not transfer accepted-connection accounting, leaving server.wait_closed() waiting after an upgrade.

This PR:

• Adds loop.runtime_info()["reactor"], sourced from that loop's constructed driver, including fallback to mio/epoll. It is None before first run and after close, remains available while stopped, and does not initialize/probe a runtime. build_info() retains its compile-time meaning for compatibility.
• Implements await writer.start_tls(), including drain, client/server inference, hostname and handshake/shutdown timeout forwarding, and replacement of writer/reader/protocol transports.
• Prevents a second server callback during transport replacement. Handler errors/cancellation close the current TLS transport.
• Transfers server connection accounting across the plaintext/TLS handoff, releasing it on failure or final closure.
• Finishes stream waiters after failed/cancelled upgrades and disposes of late handshake results without reviving a cancelled connection.
• Adds API documentation, typing coverage, runtime lifecycle tests, real STARTTLS comparisons, and deterministic late-completion tests.

Validation performed locally on Linux, CPython 3.14.7, Rust nightly-2026-09-25, and uvloop 0.23.0:

• The new runtime API test and native STARTTLS round-trip test both fail against base f16e637 with AttributeError, reproducing both missing APIs.
• Targeted runtime/STARTTLS tests: 15 passed, 14 deselected. Encrypted bidirectional traffic and EOF pass with asyncio, rsloop and uvloop, with both client-first and server-first handshake ordering. Native tests cover handler failure/cancellation, handshake failure, cancellation cleanup, late results and server shutdown.
• Full default Python selection: 284 passed, 2 failed, 3 skipped, 98 deselected. Both failures are AF_UNIX PermissionError (EPERM), also reproduced on the base checkout.
• Rust default: 320 passed, 15 failed. Rust --all-features: 416 passed, 19 failed. Failures encounter EPERM creating io_uring drivers or Unix sockets in this environment; these are not green full-suite runs.
• cargo clippy --all-targets --all-features -- -D warnings, cargo fmt --check, Ruff on changed Python files, focused Pyright and git diff --check passed.
• Observed runtime diagnostics: build_info()["reactor"] == "io_uring", but runtime_info()["reactor"] == "mio" while running/stopped, and None before initialization/after close.

Actual io_uring execution, macOS, Windows, other Python versions and free-threaded builds were not run locally. Differential tests skip stdlib STARTTLS on Python 3.10 and only pass ssl_shutdown_timeout where that reference API supports it. No new performance claim is made for this API addition.