[harbinresearcher]

Fixes #79.

arginfo() raises TypeError for ordinary unhashable callable instances (for example, dataclass functors) before reaching its existing bound-__call__ cache fallback. arginfo.is_cached() fails on the same initial instance lookup.

Treat an unhashable instance key as a reason to use that existing fallback. Keep the current cache keys and result reuse, and clarify the cache docstring. This also restores the documented None result for unhashable noncallables. Generic-function registration is outside this change.

Four new regression cases cover the bound signature, defaults, keyword-only arguments, annotations, cache state and identity, and list/dict noncallables. All four fail before the fix.

Validation:

• Windows CPython 3.12.14: full suite 194 -> 198 passed; configured statement coverage remains 100%.
• Linux CPython 3.11.17 and 3.14.8: full suite 194 -> 198 passed on each.
• Both pristine and patched Linux trees pass the project's tox -e py311,py314,coverage,lint,docs,mypy,pyright,perf, including the 100% coverage gate, Ruff check/format, strict Sphinx HTML/doctests, and both type checkers' configured 3.11-3.15 targets.
• Isolated sdist/wheel build succeeds; the installed wheel passes all 61 arginfo tests.
• Independent automated read-only review found no actionable issues. PyPy, macOS, and the remaining runtime matrix were not run locally.

Disclosure: this contribution was investigated, implemented, tested, and reviewed autonomously with AI under the contributor's authorization. The independent review was also automated, not human approval.

[henri-hulski]

Looks good to me. Thank you for the contribution.