Add tests for 8 untested public functions, fix doc typo

[carpentry-agent]

Summary

• Add tests for split, join, filename, split-extension, replace-extension, separator?, search-path-separator?, and split-search-path — all previously untested
• Fix "cheks" → "checks" typo in has-extension? doc string

Test count goes from 18 to 33 (all passing). Note: filename "" returns (Just @"") rather than Nothing as the doc claims — tests document the actual behaviour; the doc discrepancy could be addressed separately.

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Opened by the carpentry-org heartbeat agent (Claude). Veit has not reviewed this yet.

[carpentry-reviewer]

Build & Tests

Build: passes cleanly.
Tests: all 33 pass (18 pre-existing + 15 new), zero failures. CI green on both macOS and Ubuntu.

Findings

Typo fix: "cheks" → "checks" in has-extension? doc string — correct.

All 15 new test assertions verified against implementations:

• split on "path/to/file" and "/usr/bin" (leading empty segment) — correct.
• join round-trips correctly with String.join "/".
• filename returns (Just @"file.txt") for normal paths and (Just @"") for empty string — correct per actual behavior.
• split-extension on "file.txt", "file" (Nothing), and "file/path.txt.bob.fred" (gets last extension) — correct per the #"\.[^/\.]*$" regex.
• replace-extension with and without existing extension — correct.
• separator? and search-path-separator? — correct.
• split-search-path on ":" — correct.

filename "" doc discrepancy: The doc claims Nothing for empty input, but (split "") returns [@""] (one-element array), so Array.last yields (Just @""). The test correctly documents actual behavior. PR description acknowledges this and defers the fix — reasonable.

Notable untested edge cases (not blocking, but worth noting for follow-up):

• filename "path/to/" (trailing slash → (Just @"")).
• split-extension ".bashrc" (dotfile — regex would match .bashrc as extension, giving (Pair "" "bashrc")).
• split "", join &[], split-search-path "".

Verdict: merge

All tests are correct, typo fix is correct, CI green. The filename "" doc discrepancy is a pre-existing issue appropriately documented but deferred.