Audit batches 4+5: test hardening (P-6/R-3/X-2) + DX docs (X-3/X-4/X-5)

[lesnik512]

Summary

Batches 4 + 5 of the 2026-06-14 deep audit — the remaining actionable low-severity findings, in two commits.

Test hardening (commit 1):

• P-6 — test_compile_kwargs_is_memoized_across_resolves pins the compile-once invariant (spies on Factory._compile_kwargs; asserts one compile per provider across two resolves).
• R-3 — rewrite a singleton cache_item.cache is not UNSET white-box assert into a behavioral reopen check (clear_cache=False instance survives close and returns again after reopen).
• X-2 — test_typo_suggestion and the dependency-path test assert structured fields (exc.suggestions, exc.dependency_path) + message substrings instead of brittle full-string matches (drops the <locals>.Repostory repr and exact-indentation coupling).

DX / docs (commit 2):

• X-3 — concise class docstrings on every concrete exception, naming its public inspection attributes (IDE hover).
• X-4 — explicitly export exceptions from modern_di and add it to __all__.
• X-5 — docstring on ResolutionStep; note dependency_path is a list[ResolutionStep] in the errors doc.

Scope note

X-2/R-3 are deliberately surgical — the audit verifier defended exact-match asserts as a useful DX-regression guard for this library, so only the genuinely brittle cases were changed; the rest are kept as intentional guards. R-4/R-5/R-6 remain won't-fix (marginal internal tidiness); the A-1 nogil item stays in deferred.md. This PR closes every actionable audit finding.

Test Plan

• 209 passed, 100% coverage (just test-ci)
• just lint-ci clean (ruff + ty); mkdocs build --strict OK

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