fix(assertions): guard reflection walkers against cyclic inputs

[fredbi]

Three hand-rolled reflection traversals recursed unconditionally and would overflow the goroutine stack (a fatal, unrecoverable runtime error) when given cyclic-but-legal Go values:

• copyExportedFields (reached via EqualExportedValues): a self-referential struct such as type N struct{ Next *N } with n.Next = n recursed forever.
• isEmptyValue (Empty/NotEmpty/Zero/NotZero): a cyclic pointer chain (type P *P with p = &p) recursed forever.
• unwrapAll (error-chain formatting): an error with a cyclic Unwrap chain recursed forever.

Each walker now carries a set of values currently being visited on the recursion path and breaks back-edges, while leaving non-cyclic (including shared/diamond) inputs unchanged. unwrapAll only tracks comparable errors, since using an incomparable error as a map key would panic (mirroring how the standard library's errors.Is guards its own comparisons).

copyExportedFields is split into per-kind helpers to keep cognitive complexity within the linter threshold.

Adds recursion_cycles_test.go with regression tests for each case. The signal is simply that the tests run to completion: before the fix each case crashes the test binary with a stack overflow.

Note: the public ErrorIs/NotErrorIs path on a cyclic error chain still hangs inside the standard library's errors.Is (which walks Unwrap without cycle detection) before our formatter runs. Guarding that would mean reimplementing errors.Is and diverging from stdlib semantics, so it is left as-is; unwrapAll is hardened on its own as defense-in-depth.

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.70%. Comparing base (81253df) to head (544e90b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
internal/assertions/equal.go	92.85%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #129      +/-   ##
==========================================
+ Coverage   91.68%   91.70%   +0.01%     
==========================================
  Files          97       97              
  Lines       12623    12651      +28     
==========================================
+ Hits        11573    11601      +28     
  Misses        827      827              
  Partials      223      223              

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