Adopt kit vector encoding queue

[mariusvniekerk]

The local msgvault commit 4e989e5 Use kit encoding queue switches internal/vector/embed to the extracted go.kenn.io/kit/vector/encodingqueue helper. It is intentionally not a msgvault PR yet because it currently depends on the kit extraction being merged and consumable.

Dependency: kenn-io/kit#25

Follow-up work:

• After the kit PR lands, update msgvault to depend on the released or reachable kit module version instead of the local replace directive.
• Keep msgvault-specific generation selection in internal/vector/embed/enqueue.go; only the generic queue mechanics should live in kit.
• Re-run go test -tags "fts5 sqlite_vec" ./internal/vector/... ./internal/scheduler ./cmd/msgvault/cmd and go vet ./... before opening the msgvault PR.

Validation already run on the local msgvault commit: go test -tags "fts5 sqlite_vec" ./internal/vector/... ./internal/scheduler ./cmd/msgvault/cmd; go vet ./....

^{generated by a clanker}