Add a simple async API

[puddly]

This PR introduces serialx.async_serial_for_url, a simple async API:

import serialx

async with serialx.async_serial_for_url(
    "/dev/serial/by-id/port", baudrate=115200,
) as serial:
    data = await serial.readexactly(5)
    serial.write(b"test")
    await serial.flush()

    await serial.set_modem_pins(rts=True, dtr=True)
    pins = await serial.get_modem_pins()
    assert pins.rts is serialx.PinState.HIGH

This makes migration of existing sync code straightforward, as often times even simple scripts graduate to needing background tasks, timeouts, and so on. I've added some documentation to steer users to either use this async API or to use the low-level serialx.create_serial_connection API. The intermediate serialx.open_serial_connection returning StreamReader and StreamWriter is clunky to use and error-prone, especially when it comes to open/close.

Many unit tests have also been simplified to take advantage of this simpler API.

[codecov]

Codecov Report

❌ Patch coverage is 96.92308% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.08%. Comparing base (b4a65e7) to head (9e2c313).
⚠️ Report is 3 commits behind head on dev.

Files with missing lines	Patch %	Lines
serialx/async_serial.py	96.19%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #79      +/-   ##
==========================================
+ Coverage   90.71%   91.08%   +0.37%     
==========================================
  Files          22       22              
  Lines        3372     3748     +376     
==========================================
+ Hits         3059     3414     +355     
- Misses        313      334      +21     

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[Copilot]

Pull request overview

This PR introduces a new simplified asyncio-facing API (serialx.async_serial_for_url) that returns an AsyncSerial object with a sync-like method surface, and updates tests + documentation to prefer this higher-level async interface over the existing (StreamReader, StreamWriter) pattern.

Changes:

• Added AsyncSerial and the async_serial_for_url() factory, and exported them from serialx.
• Refactored tests to use AsyncSerial (including cross-loop ESPHome scenarios) and simplified test helpers.
• Updated/expanded docs to steer users toward the simplified async API while keeping the streams/transport APIs documented.

Reviewed changes

Copilot reviewed 17 out of 17 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
serialx/async_serial.py	Introduces AsyncSerial and async_serial_for_url(); implements read/write/flush and modem-pin helpers.
serialx/__init__.py	Exports AsyncSerial and async_serial_for_url from the package root.
serialx/common.py	Adds ConnectKwargs TypedDict and updates BaseSerialTransport.connect/_connect typing to use Unpack.
serialx/descriptor_transport.py	Adjusts _connect signature to accept path + ConnectKwargs.
serialx/platforms/serial_esphome.py	Updates transport _connect signature and forwards path explicitly.
serialx/platforms/serial_rfc2217/__init__.py	Updates transport _connect signature and forwards path explicitly into RFC2217Serial.
serialx/platforms/serial_win32.py	Updates transport _connect signature and typing around forwarded connect kwargs.
tests/common.py	Replaces stream reader/writer helpers with async_create_serial_pair() yielding AsyncSerial objects.
tests/test_async_transports.py	Migrates tests to AsyncSerial and the new serial-pair helper.
tests/test_serial_esphome.py	Migrates ESPHome tests from streams API to AsyncSerial, including cross-loop coverage.
README.md	Documents the simplified async API and keeps the streams/transport alternatives.
docs/usage.md	Adds a “simple async API” section and positions streams/transport as alternatives.
docs/quickstart.md	Updates quickstart async section to use async_serial_for_url.
docs/index.md	Adds a new how-to page entry for async serial.
docs/how-to/async-serial.md	New how-to describing patterns and tradeoffs for the simplified async API.
docs/how-to/pyserial-migration.md	Adds guidance for migrating sync serial_for_url code to the new async API.
docs/api/platforms/posix.md	Removes an extra autofunction block from the POSIX docs page.

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