[krafczyk]

Summary

Fix the remaining constructor-diagnostic gap in #6: Definition(...).build() retained a misspelled keyword only in the exception cause, so displaying the primary exception lost the actionable detail.

For a closed constructor signature, identify a known unexpected keyword from the declared parameters and supplied keyword names, then retain it in the existing SignatureError.slot field. For example:

Before: Signature normalization failed: argument binding failed.
After:  Signature normalization failed: argument binding failed at chlid.

The machine-readable reason remains "argument binding failed", and the original Python TypeError remains the cause. Diagnostics do not render argument values or parse raw exception text. The change is constructor-only; ordinary function binding and valid variadic keyword names retain their existing handling. Update the public signature documentation and release notes.

Fixes #6.

Verification

• Reproduced the public Definition.build() regression before applying the fix.
• 38 signature tests passed, including five new cases covering the primary message/slot, preserved cause, non-renderable argument values, ordinary function behavior, accepted variadic keywords, and malformed non-string keys.
• 94 affected tests passed across signatures, semantic argument binding, object/definition creation, signature consumers and documentation links.
• 4,217 passed, 39 skipped in the required representative good-enough suite (277 process-state tests plus 3,940 ordinary tests). All five new cases are included in the routine profile.
• Changed-file fatal lint and whitespace checks passed.

Local verification used the existing Python 3.12.13 big_env environment with CPU confinement and single-thread native limits. No dependency or persistent-format change is required.