Add bounded Watchman diagnostics and large-tree regression probe

[schickling-assistant]

Goal

Add bounded diagnostics and the remaining regression coverage for composed-workspace Watchman behavior after the core fix in #1328.

Scope

• Report the selected Watchman root.
• Report the loaded root configuration.
• Report watched-directory count.
• Report recrawl count.
• Report Watchman registration latency.
• Add a real-Watchman regression probe with a large unrelated ancestor or sibling tree and verify that its size does not affect the composition watch.

Context

PR #1328 fixed generated .watchmanconfig ownership, concrete ignore paths, exact-root reconciliation, and rollback/recovery lifecycle. The PR explicitly deferred diagnostics. This follow-up carries the remaining diagnostics and probe work from #1293.

Refs #1293

Posted on behalf of @schickling

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