Optimize slcan serial read and buffer processing

[YeWenxuan64]

Refactor reading from serial to improve performance by using C level's bytearray.find() to replace for iteration.

Summary of Changes

Key changes:

• Removed the Python-level for i, byte in enumerate(buffer): if byte in (ok, err): ... loop
• Replaced with two buffer.find(byte) calls at C level for terminator detection

Related Issues / Pull Requests

• None

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactoring
• Other (please describe):

Checklist

• I have followed the contribution guide.
• I have added or updated tests as appropriate.
• I have added or updated documentation as appropriate.
• I have added a news fragment for towncrier.
• All checks and tests pass (tox).

Additional Notes

Method	vs bytearray.find (v5)
v1 for loop	~2–4× slower
v2 single-pass local	~2–4× slower
v3 filter+next	~3–6× slower
v4 bytes.find	~1.5–2× slower (copy overhead)
v5 bytearray.find	1.0× (baseline)

I could also provide performance testing scripts (generate by AI) if you need.

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request

[YeWenxuan64]

A benchmark generate by AI
benchmark_read_methods.py

[YeWenxuan64]

@Mergifyio queue

[mergify]

queue

☑️ Command disallowed due to command restrictions in the Mergify configuration.

Details

• sender-permission >= write