Replace mattn/go-sqlite3 with modernc.org/sqlite (drop cgo dependency)

[e7217]

Motivation

EDG Core currently depends on github.com/mattn/go-sqlite3 for its metadata store. go-sqlite3 is a cgo wrapper around the SQLite C library, which means:

1. Windows builds require a gcc toolchain (mingw-w64 or equivalent). Default go build on Windows ships without cgo, producing a binary that fails at runtime:

   Failed to create store: ... go-sqlite3 requires cgo to work. This is a stub
   

   Field engineers spinning up edg-core on a Windows IPC, devs running quick smoke tests, or CI runners that don't have gcc installed all hit this wall.

2. Cross-compilation is awkward. GOOS=linux go build from a Windows host without a Linux gcc cross-toolchain silently produces the same broken stub.

3. CGO_ENABLED=1 leaks into downstream Dockerfiles, CI matrices, and release pipelines. Every contributor environment needs to be cgo-capable.

Discovered during Beckhoff adapter smoke testing on Windows — edg-core started NATS successfully but exited immediately when initializing the metadata store.

Proposed change

Replace the SQLite driver with modernc.org/sqlite, a pure-Go transpilation of the SQLite C source. No cgo required.

Scope

• go.mod: drop github.com/mattn/go-sqlite3, add modernc.org/sqlite
• internal/core/store*.go (and anywhere else using sql.Open("sqlite3", ...)): change driver name to "sqlite"
• golang-migrate driver: swap github.com/golang-migrate/migrate/v4/database/sqlite3 → .../sqlite (the modernc-compatible package)
• PRAGMA / DSN compatibility audit (foreign keys, WAL mode, busy timeout) — modernc.org/sqlite follows SQLite-standard PRAGMAs but some _pragma= DSN extensions differ
• Run full test suite locally and in CI (Windows + Linux)
• Re-test migration up/down paths against an existing metadata.db produced by mattn/go-sqlite3

Out of scope

• Adapter SDK code (no sqlite usage)
• Migration of any production deployments — the on-disk SQLite format is identical, so existing metadata.db files keep working.

Risk

• Performance: modernc.org/sqlite is ~10-30% slower than cgo in microbenchmarks. EDG Core uses sqlite for asset metadata (low write rate, mostly read), so this should not matter in practice. Worth a quick benchmark before merging.
• Subtle SQL behaviour differences: Both drivers target the same SQLite engine version, so feature coverage matches; differences are usually in driver-level features (custom functions, connection hooks) which EDG Core doesn't currently use.
• golang-migrate driver maturity: The modernc.org-backed migrate driver is less battle-tested than the mattn variant. Verify by running all existing migrations on a fresh DB.

Acceptance criteria

• go build ./... succeeds on Windows with no gcc toolchain installed.
• edg-core -config deploy/configs/core/config.dev.yaml starts cleanly on Windows and creates metadata.db.
• All existing unit/integration tests pass on Linux and Windows.
• A fresh metadata.db from this branch is byte-readable by sqlite3 CLI (rules out exotic file format issues).

Workaround until then

Install mingw-w64 (e.g. via winget install BrechtSanders.WinLibs.POSIX.UCRT) and build with CGO_ENABLED=1. This is what we're doing in the Beckhoff smoke test setup currently.