refactor: introduce database provider abstraction, isolate MSSQL layer

- Add provider contract (src/providers/provider.js): query() -> {rows,
  rowsAffected, insertId}, callback transaction(fn), exec() for stored
  procs, initialize/purge lifecycle
- Add MssqlProvider (src/providers/mssql/index.js) wrapping an mssql
  ConnectionPool with normalized results, tx commit/rollback and
  constraint-error classifiers (unique 2627/2601, FK 547)
- LibDashStore constructor now takes a provider instead of a pool
  (breaking); adds LibDashStore.create() async factory; keeps pool as
  transitional bridge until store modules are converted (see TODO.md)
- Move MSSQL-specific code into the provider: src/sql/* ->
  src/providers/mssql/queries/*, src/schema.js ->
  src/providers/mssql/schema.js, purge SQL -> purgeSql.js
- Update scripts, generate-schema-graph.js and package.json exports
  (./providers/mssql, ./providers/provider)
- Add TODO.md with the remaining provider/SQLite migration plan

No SQL or behavioral changes.

src/providers/provider.js

@@ -0,0 +1,40 @@
+/**
+ * Database provider contract for LibDashStore.
+ *
+ * A provider wraps a concrete database driver and exposes:
+ *   - query(sql, params)            -> { rows, rowsAffected, insertId }
+ *   - transaction(fn)               -> runs fn(runner) in a tx, commit/rollback
+ *   - exec(proc, {inputs,outputs})  -> stored procedure call (optional capability)
+ *   - initialize() / purge()        -> schema lifecycle
+ *   - pool                          -> the underlying driver handle (transitional)
+ *
+ * @typedef {Object} QueryResult
+ * @property {Array<Object>} rows       Result rows (mssql `recordset` equivalent).
+ * @property {number} rowsAffected      Number of affected rows (mssql `rowsAffected[0]`).
+ * @property {number|null} insertId     Last inserted id when applicable, else null.
+ *
+ * @typedef {Object} DatabaseProvider
+ * @property {(sql: string, params?: Object<string, *>) => Promise<QueryResult>} query
+ * @property {(fn: (runner: DatabaseProvider) => Promise<*>) => Promise<*>} transaction
+ * @property {() => Promise<void>} initialize
+ * @property {() => Promise<void>} purge
+ * @property {*} pool
+ */
+
+/**
+ * Checks whether an error is a unique-constraint violation.
+ * @param {unknown} err
+ * @returns {boolean}
+ */
+export function isUniqueConstraintError(err) {
+  throw new Error('Not implemented: isUniqueConstraintError');
+}
+
+/**
+ * Checks whether an error is a foreign-key-constraint violation.
+ * @param {unknown} err
+ * @returns {boolean}
+ */
+export function isForeignKeyConstraintError(err) {
+  throw new Error('Not implemented: isForeignKeyConstraintError');
+}