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.

scripts/test-api.js

@@ -4,6 +4,7 @@ import sql from 'mssql';
 import chalk from 'chalk';
 import * as p from '@clack/prompts';
 import LibDashStore from '../src/index.js';
+import { MssqlProvider } from '../src/providers/mssql/index.js';
 
 const config = {
   server: process.env.MSSQL_SERVER,
@@ -2004,7 +2005,7 @@ async function main() {
   spin.message('Initializing schema…');
   let client;
   try {
-    client = await new LibDashStore(pool);
+    client = await new LibDashStore(new MssqlProvider(pool));
     spin.stop(chalk.green('Ready'));
   } catch (err) {
     spin.stop(chalk.red('Initialize failed'));