test: add gloss-core test suite (red phase)

The commit lands eight per-function test files and a shared testutil. 32 tests across Normal/Boundary/Error categories cover the public API (lookup, save, list, find-buffer-position) and the internals that need observable behavior tests (mtime invalidation, corrupt-file resilience, alphabetical insert, first-call file creation).

All 32 fail with void-function on the gloss-core symbols. That is the intended red-phase signal. The next commit lands the implementation that turns them green.

testutil-gloss provides a with-temp-glossary macro. It binds gloss-file to a temp file, resets the cache before and after, and cleans up the visiting buffer.

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