Docs: Report findings for issue #2177 and add regression tests

[google-labs-jules]

I have confirmed that the private_bounds lint failure reported in #2177 is likely a rustc bug related to hygiene in mixed macro contexts.

Findings:

1. Reproduction: The private_bounds lint triggers when #[derive(KnownLayout)] is used inside a macro_rules! macro. This is reproduced in zerocopy-derive/tests/issue_2177.rs (which passes only because of the existing #[allow(private_bounds)] suppression).
2. Control Case: Manually expanding the code generated by KnownLayout inside a macro_rules! macro (without using the proc-macro) does not trigger the lint. This is demonstrated in zerocopy-derive/tests/issue_2177_control.rs. This isolates the issue to the interaction between proc_macro generated spans and macro_rules! expansion, confirming it's not a logic error in the generated code itself.

I have verified that the current main branch (with the workaround) passes these tests. I am committing these tests to prevent regression and document the behavior.

Command Log:
To reproduce the failure (temporarily revert #[allow(private_bounds)] in zerocopy-derive/src/lib.rs):
yes | ./cargo.sh +nightly test -p zerocopy-derive --test issue_2177

To verify the control case passes (no changes needed):
yes | ./cargo.sh +nightly test -p zerocopy-derive --test issue_2177_control

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PR created automatically by Jules for task 6073541928888338093 started by @joshlf

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.14%. Comparing base (7b1260f) to head (fca2a8b).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2787   +/-   ##
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  Coverage   88.14%   88.14%           
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  Files          20       20           
  Lines        5450     5450           
=======================================
  Hits         4804     4804           
  Misses        646      646           

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