Route OSD notifications through overlay with mpv fallback

- Add main/renderer overlay notification pipeline with loading-state support
- Preserve notificationType semantics across osd/system/both/none routing
- Update plugin fallback behavior, docs/config examples, and regression tests

backlog/tasks/task-166 - Replace-OSD-notifications-with-overlay-notifications-and-runtime-fallback.md

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+---
+id: TASK-166
+title: Replace OSD notifications with overlay notifications and runtime fallback
+status: In Progress
+assignee: []
+created_date: '2026-03-16 00:00'
+updated_date: '2026-03-16 00:00'
+labels:
+  - overlay
+  - notifications
+  - ux
+dependencies: []
+references:
+  - /home/sudacode/.t3/worktrees/SubMiner/t3code-c5d9c2f7/src/main.ts
+  - /home/sudacode/.t3/worktrees/SubMiner/t3code-c5d9c2f7/src/renderer/renderer.ts
+  - /home/sudacode/.t3/worktrees/SubMiner/t3code-c5d9c2f7/plugin/subminer/process.lua
+priority: medium
+---
+
+## Description
+
+<!-- SECTION:DESCRIPTION:BEGIN -->
+
+Replace SubMiner's mpv OSD-first notification flow with overlay-native notifications. The visible overlay becomes the primary in-app notification surface, with fallback to mpv OSD only when the overlay is unavailable, hidden, disabled, or failed to load. Electron/system notifications remain unchanged. Existing notification copy and loading states should be refreshed to fit the richer overlay surface, especially spinner-driven loading states.
+
+<!-- SECTION:DESCRIPTION:END -->
+
+## Acceptance Criteria
+
+<!-- AC:BEGIN -->
+
+- [ ] #1 Main-process logical OSD notifications render on the overlay when the visible overlay is available and visible.
+- [ ] #2 Logical OSD notifications fall back to actual mpv OSD when the overlay is hidden, disabled, unavailable, or fails to load.
+- [ ] #3 Notification-type config semantics remain intact: `osd` uses the overlay-or-OSD channel, `system` remains Electron notifications, `both` uses both channels, and `none` suppresses notifications.
+- [ ] #4 Overlay notifications support richer presentation for loading/progress states, including a spinner/persistent loading state.
+- [ ] #5 Regression coverage verifies routing, renderer behavior, and fallback handling.
+
+<!-- AC:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+
+Plan:
+1. Add failing tests for a pure main-process overlay notification router and a renderer overlay notification controller.
+2. Add overlay notification IPC/types plus renderer DOM/controller/CSS for styled toast notifications and spinner handling.
+3. Route existing logical OSD callsites through the new overlay-or-OSD fallback seam in main.
+4. Trim plugin-side direct OSD usage down to true fallback/error cases.
+5. Verify with targeted runtime-compat/core lanes, then escalate if runtime behavior claims remain unproven.
+
+<!-- SECTION:NOTES:END -->