Fix various issues for 5.x release

[dereuromark]

Summary

Fixes various issues identified during the 5.x release preparation:

• Fix README.md coverage badge pointing to 3.x branch instead of 5.x
• Update MigrationHelper comments to remove outdated phinx-related TODOs (phinx was removed in 5.x)
• Add comprehensive database-specific limitations documentation covering SQL Server, SQLite, PostgreSQL, and MySQL/MariaDB
• Fix PHPStan issues by adding proper @var type hints for Bake event subjects
• Update PHPStan baseline to remove the 3 now-fixed loadHelper() errors

Changes

Documentation

• writing-migrations.rst: Added new "Database-Specific Limitations" section documenting:
  • SQL Server: check constraints, table comments, INSERT IGNORE not supported
  • SQLite: foreign key names, table comments, check constraint modifications, partitioning not supported
  • PostgreSQL: KEY partitioning not supported
  • MySQL/MariaDB: insertOrUpdate conflict columns behavior, MariaDB GIS issues

Code Quality

• BakeMigrationCommand.php, BakeMigrationDiffCommand.php, BakeMigrationSnapshotCommand.php: Added proper @var \Bake\View\BakeView annotations to fix PHPStan errors about undefined loadHelper() method
• MigrationHelper.php: Replaced outdated phinx TODOs with accurate comments explaining precision/scale conversion between CakePHP TableSchema and SQL standard formats
• phpstan-baseline.neon: Removed 3 fixed errors from baseline

Related

Part of the preparation for 5.x major release (#995)