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/store/company/create.js

@@ -1,11 +1,11 @@
-import { POS_GROUP_NAME } from '../../sql/entity.js';
-import { CREATE } from '../../sql/company.js';
-import { CREATE as CREATE_CATEGORY } from '../../sql/category.js';
+import { POS_GROUP_NAME } from '../../providers/mssql/queries/entity.js';
+import { CREATE } from '../../providers/mssql/queries/company.js';
+import { CREATE as CREATE_CATEGORY } from '../../providers/mssql/queries/category.js';
 import {
   CREATE_WITHOUT_COMPANY as CREATE_CUSTOMERGROUP,
   SET_COMPANY as SET_CUSTOMERGROUP_COMPANY,
-} from '../../sql/customergroup.js';
-import { ADD as ADD_COMPANYUSER } from '../../sql/companyuser.js';
+} from '../../providers/mssql/queries/customergroup.js';
+import { ADD as ADD_COMPANYUSER } from '../../providers/mssql/queries/companyuser.js';
 import getByName from './getByName.js';
 import langEnsure from '../lang/ensure.js';
 import taxEnsure from '../tax/ensure.js';