[PP-2] Get rid of 2-phase kernel boot in installer

Problem/Motivation

The Drupal installer uses a 2-phase kernel boot with an in-process kernel reboot:

1. Phase 1 (InstallerServiceProvider): In-memory stubs (no DB services). Used from start through install_base_system.
2. Kernel reboot in drupal_install_system(): shutdown() => rebuildContainer() => boot() to switch providers.
3. Phase 2 (NormalInstallerServiceProvider): Database-backed services for the remainder.

This adds complexity: two service provider classes, an in-process kernel reboot, and the base_system_verified flag to pick between them.

Main blocker: InstallStorage is read-only (throws on write). System module install needs to WRITE config. So the kernel reboots to switch to DatabaseStorage first.

There's prerequisites (last 2 probably optional)

- #3335756: Drop sequences table in Drupal 12
- #2719315: Try to install system module like any other module
- #2934063: Remove the workaround in \Drupal\Core\Installer\ExtensionListTrait::setPathname()

Proposed resolution

Create InstallerFallbackConfigStorage + merged service provider

ModuleInstaller::install() already triggers a container rebuild internally. If we set base_system_verified = TRUE before calling module_installer->install(['system']), the rebuild picks up the flag and registers full DB-backed services (without an explicit kernel reboot).

Merge NormalInstallerServiceProvider into InstallerServiceProvider and deprecate first one.
Remove kernel reboot from drupal_install_system()

Remaining tasks

- possibly listAll() will need deduplication (DB and install stroages)
- dig URL generation and Routing
- find out risks & edge cases
- probably should wait for D12.1 for deprecation

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet