Retire obsolete Decent Scale Off plugin

[ODevStudio]

Summary

• retire Decent Scale Off as an inert compatibility shim
• hide the obsolete plugin from the plugin list
• migrate persisted decentscale_off enablement out of settings
• add regression coverage for the shim, visibility, and migration

Root cause

The plugin replaced the core scale_disable_lcd procedure and delayed the scale power-off write by three seconds. The normal sleep path disconnects the scale after one second, so the connection could close before the plugin sent the command. Core app code now owns this behavior and queues power-off before disconnecting.

Impact

Existing installations receive the inert shim through the updater, while new users cannot enable the obsolete plugin. Persisted enablement is removed and saved once, preventing an older plugin copy from reactivating after a channel downgrade. The core keep_scale_on behavior remains unchanged.

Validation

• tests/decentscale_off.test.tcl: 3 passed
• migration removes only known retired plugins and is idempotent
• git diff --check
• verified the core sleep path orders scale_disable_lcd before the delayed disconnect
• hardware validation not run