tests: optional follow-ups — single-file slack import test; semantic-completeness property

[edheltzel]

Source: PR #56 re-review + PR #61 review (both optional tier, promoted to tracking on filing sweep).

1. Single-file slack test (PR refactor: conversation imports behind internal source-adapter registry #56): the directory-input test pins the rootPath miswire, but a dropped rootPath destructure is coincidentally masked by the dirname fallback in the nested case — one single-file slack import test closes that residual mask.
2. Semantic-completeness property (PR test: add property-based tests for export and dedup invariants (issue #44 phase 2) #61): the dedup semantic-pass property is soundness-only — a dropped-pairs regression would pass; a two-record exactly-determined property (one pair above threshold MUST be found) closes it.