Improve file registry performance and consistency

## Problem
Our `WatchedFiles` (in `src/shared/fs/watchedFiles.ts`) implementation has two major problems:
* It consumes a lot of system resources on extension activation
* File detection is unreliable, introducing race conditions and caching bugs
* example #3045)

### Related

- #1546
- #2623

## Expected behavior
The Toolkit should defer expensive operations until they are needed. Lazy evaluation should be favored over file watching.

Start with src/shared/fs/watchedFiles.ts and review related issues #1546, #2623, and example #3045 to understand the resource, race-condition, and caching problems. Define how lazy evaluation should replace or reduce file watching, then verify that extension activation defers expensive work and file detection remains consistent.

Written by the indexing model from the issue text.