fix(core): name the language in per-language config key errors

[ElbertePlinio]

Closes #41

nested_keys in crates/core/src/config.rs now takes the path of its parent object. The per-language call passes languages.<name>, so an unknown per-language limit key is reported as languages.<name>.limits.<key> and a non-object per-language limits as languages.<name>.limits in <file> must be an object. Top-level calls pass an empty parent, so their messages are unchanged.

Requirement	Verified by	Scenario	Result	Evidence
The error names languages.<name>.limits.<key>	automated test + CLI QA	{"languages":{"go":{"limits":{"widgetdepth":1}}}}; two languages with the typo in rust only; dotted language name go.x	pass, exit 2 with the full path	unknown_language_limit_key_names_full_path, unknown_language_limit_key_names_the_language_with_the_typo; QA 01, 02, 12
Non-object per-language limits names its path	automated test + CLI QA	{"languages":{"go":{"limits":1}}}	pass	non_object_language_limits_names_full_path; QA 04
Top-level limits.<key> errors are unchanged	automated test + CLI QA	limits.widgetdepth, non-object limits, tests.pattern, hook.blocks, languages.go.limit, top-level limit, valid overrides, empty per-language limits	byte-identical to main, including exit codes	unknown_top_level_limit_key_has_no_language_path, unknown_nested_key_names_full_path; QA 03, 05 to 11, 13
A test covers the per-language case	automated test	the four new unit tests above	pass	crates/core/src/config.rs
Tests, lint, format, coverage, PickCheck	automated	cargo test --workspace --locked --all-targets, cargo clippy ... -D warnings, cargo fmt --check, cargo run -- check crates, cargo llvm-cov --fail-under-lines 94 (94.71%), pickcheck check --base origin/main --fail-on new,worsened,unmatched (0 violations), all rerun on ab4c20f	pass	CI on this PR

QA: a CLI pass ran release builds from main (4de9c09) and this branch over a scratch Git repo in 13 .pickcheck.json scenarios, plus --changed and --base HEAD --format json with the two-language typo. Only the four per-language path cases differ from main. The evidence stays local and outside the repo.

Review: independent xhigh reviews by Astra and Fable at d302b62. Astra found nothing. Fable found that docs/releases/UNRELEASED.md lacked a Changes line for #41, and that a negative assertion on "languages" could fail spuriously if the temp directory path contained that word. ab4c20f fixes both, and the Fable recheck confirmed it. A suggested shared helper for the prefix join was declined because it adds a new function for two call sites. ab4c20f changes only a test and the release draft, so the CLI QA was not rerun.

Follow-ups found during this work: #43 (non-object languages.<name> error omits the config file) and #44 (README coverage floor differs from CI).