Add PyMemoryView::from_owned_buffer for zero-copy memoryview creation

[alex]

Adds a new method to create a Python memoryview that exposes a read-only view of byte data owned by a frozen PyClass instance without copying. This is useful for libraries like pyca/cryptography that need to expose internal buffers efficiently.

The method uses PyBuffer_FillInfo + PyMemoryView_FromBuffer to create a memoryview backed by the owner's data, with the owner kept alive via the buffer's obj reference.

Safety is enforced at compile time:

• T: PyClass<Frozen = True> prevents mutation that could invalidate pointers
• for<'a> FnOnce(&'a T) -> &'a [u8] ensures the slice borrows from T or is 'static

Closes #5871

https://claude.ai/code/session_01EEP1DaqJwHGCoNufi2JT9H

[codspeed-hq]

Merging this PR will not alter performance

✅ 105 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing alex:claude/implement-pyo3-api-5871-kzVaG (7f86fc8) with main (df36c71)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[alex]

netlify stuff looks unrelated?

[davidhewitt]

Indeed, hopefully #5939 will resolve that.

[davidhewitt]

#5941 might make the call_method_one_arg benchmark more stable...

[alex]

No luck with netlify...

[davidhewitt]

#5945 has finally got things green, one more rebase / merge should fix.