feat(refid): add raw-SQL sqlite backend

[sthanikan2000]

Summary

• Adds refid/store/sqlite, a second refid.SequenceStore implementation using the pure-Go modernc.org/sqlite driver (no CGO). Same shape as refid/store/postgres: DefaultTableName, Option, WithTableName, New, Migrate, single atomic upsert-and-increment Next.
• SQLite only allows one writer at a time (whole-database file lock, not per-row like Postgres), so New guards Next with an in-process sync.Mutex rather than touching the caller's *sql.DB connection-pool configuration. The package doc's "Concurrency" section spells out exactly what that mutex covers (calls through one SequenceStore instance in one process) and what it doesn't (a second connection/process sharing the same file — configure SQLite's busy_timeout for that).
• Useful on its own as a convenient refid.SequenceStore for local dev and tests, in addition to being a real second backend.
• Stacked on refactor(refid)!: replace gorm-based postgres store with raw SQL under refid/store/postgres #180 (raw-SQL postgres, itself stacked on revert: refactor(refid)!: rename SequenceStore to Store, replace gorm postgres backend with raw SQL #179 the revert) — so this diffs cleanly against a SequenceStore-named, refid/store/postgres-shaped main once both merge in order.

Test plan

• go build ./..., go vet ./..., golangci-lint run clean
• go test -race ./... — increment, overflow-freezes-counter, invalid-table-name-returns-error-not-panic, 50-goroutine no-duplicates, and a regression test proving Next fails promptly (not hangs) when another transaction on the same *sql.DB holds SQLite's write lock
• Pre-push hook's full monorepo go test -race ./... passes

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