Stop embeddings_test sharing one dense vector with every other test

[mkreyman]

Sibling of #747. Same defect class, different module.

The defect

vec(dim) was sin(i / dim) and vec(dim, seed) was sin((i + seed) / dim) — dense,
and identical across every test that asked for the same dim. That is a distance-0 clique
in the shared pgvector HNSW index: under concurrent load the graph walk cannot navigate a
clique of exact ties and non-deterministically misses exact matches (KB 4e5ccb17,
0bdadd52). It is the same reason DataCase's default MockEmbeddingClient stub keys its
vector on tenant_id instead of returning a constant.

Not "dead tuples crowd out the live row" — KB c6fd1e5d lists that as a dead theory,
disproved by probe (pgvector skips invisible tuples and keeps scanning past 2000 rolled-back
ties). That article opens by noting the family has been misdiagnosed twice, each time
costing reverted fixes, so the reasoning is recorded next to the helpers.

The exposed assertion

The dimension-migration test at the end of the file stores a row with vec(768) and queries
with the same vector through VectorSearch.index_safe_dimension_knn_base, asserting exactly
one hit. A trapped walk returns [] there — which reads as a broken read path rather than
as this.

The fix

Both helpers ride test_vec/2, so this module's rows sit on its own sparse dimensions
(seeded per test by Process.put(:test_vec_axis, …) in DataCase.setup). vec/2 keeps its
N-distinct contract by scaling a component on the test's own orthogonal window — no call
site depends on a particular similarity between those vectors, only that they are distinct
and findable.

Free, like #747: this is not the @moduletag :vacuum_vector_indexes route, which measured
+50% on the whole suite for one module and 18.6x for 24.

Verification

Full gate green through the pre-commit hook: 7827 tests, 0 failures (86 excluded), credo
--strict clean, dialyzer clean. The file's own 34 tests pass.

Review

Reviewed inline and against the KB, which is what supplied the mechanism and ruled out the
one I would otherwise have written down. Two helper functions in one test file.