Bitrix modules

Baseline: main 23.0+ · Verified: main 26.800.0

Task	Read
New module: id, skeleton, `install/index.php`, version, lang, uninstall	`rules/installer.md`
Tables, events, agents in `install/migrations/*.php` (main 26.600+)	`rules/install-migrations.md`
Options page, `default_option.php`, PSR-4 autoload, module routes	`rules/options-autoload.md`

Invariants

Module dir /local/modules/<vendor>.<module>/; id lowercase vendor.module; installer class vendor_module; namespace Vendor\Module (one-word id mymodule → Bitrix\Mymodule).
Nothing works until the module is registered in b_module: Loader::includeModule() returns false, and module .settings.php (services, controllers, console, messenger) is not read.
Install/uninstall are symmetric: every table, handler, agent and option you create has a removal path; re-running install must not fail.
Event registration uses kernel parameter names (fromModuleId:) or positional args; handler signature depends on the dispatcher — bitrix-events.
Module routes are required from /local/routes/web.php; a routing section in module .settings.php is ignored (bitrix-routing).
Project data/content migrations (iblocks, options per environment) belong to bitrix-sprint-migration, not to the installer.