Compile prerequisite extensions before starting watchers in watch-extensions

[softwarenerd]

Problem

Ext - Build Task is red almost all the time

Fix

Apply the same prerequisite ordering that compile-extensions already uses to watch-extensions.

Summary

watch-extensions started every extension watcher in parallel, so extensions that consume an in-repo package's compiled out/ (currently positron-data-explorer-duckdb -> positron-data-explorer-protocol) could race: a watcher's clean step deletes out/ while a dependent is already resolving its .d.ts files. This compiles the ordered prerequisites to completion first, then starts all watchers in parallel. Since watchers are long-lived, they can't be sequenced with each other, so the prerequisites' watchers use a new watch-extension-no-clean:<name> variant that skips the clean step and preserves the out/ that was just built for their dependents. The watch stream body is extracted into a createWatchStream() helper so both variants share it; no behavior change for extensions that aren't prerequisites.

Release Notes

New Features

• N/A

Bug Fixes

• N/A

Validation Steps

Build-only change, no e2e coverage applies.

1. From a clean tree (git clean -xfd extensions/positron-data-explorer-*/out or equivalent), run npm run watch-extensions.
2. Verify positron-data-explorer-protocol and positron-data-explorer-duckdb compile before the remaining watchers start, and that no Cannot find module/unresolved-import errors appear for the DuckDB backend.
3. Edit a file in each of those two extensions and confirm the watcher still picks up the change and recompiles.
4. Confirm npm run compile-extensions is unchanged.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical

Note

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[jmcphers]

Feels really good to have these all reporting healthy!