Improve astuple performance for nested atomic sequences

[Johnny-Kao]

Summary

Avoid calling has() for values whose exact type is already known to be atomic when attrs.astuple() traverses a nested sequence.

The top-level path already skips attrs detection for _ATOMIC_TYPES; this applies the same shortcut to sequence elements while preserving the existing fallback for all non-atomic values.

Benchmark

Local CPython 3.14 benchmark on list[int]:

Elements	main	this PR	Speedup
10	0.914 µs	0.519 µs	1.76×
100	5.704 µs	1.931 µs	2.95×
1000	53.373 µs	16.037 µs	3.33×

A CodSpeed benchmark for nested atomic sequences is included.

Testing

• uv run pytest -q: 1407 passed, 4 skipped, 2 xfailed
• uv run pytest bench/test_benchmarks.py::test_astuple_nested_atomic -q
• uv run ruff check src/attr/_funcs.py bench/test_benchmarks.py
• uv run ruff format --check src/attr/_funcs.py bench/test_benchmarks.py
• git diff --check

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
• Tests / benchmarks cover the changed path.
• No public API or typing changes.
• Changelog fragment added.