[Foundation] FT-1: Fix cross-invocation static-state leaks (event handlers, log sink, verbosity mappings)

[ChrisonSimtian]

Summary

Fix cross-invocation static-state leaks in the engine. These are latent bugs today — they manifest whenever BuildManager.Execute runs twice in one process (test harnesses, the dotnet fallout global tool, future in-process/IDE scenarios).

Findings (file:line)

• BuildManager.cs:42 — ToolOptions.Created += … subscribed every run, never removed.
• BuildManager.cs:41 / :69 — Console.CancelKeyPress += and CancellationHandler += Finish accumulate; s_cancellationHandlers (:20) is never cleared.
• Logging.cs:214-233 — InMemorySink.Instance._logEvents accumulates; Dispose() (which clears) is never called → run 2 reports run 1's errors/warnings.
• VerbosityMappingAttribute.cs:34-46 — Mappings is a LookupTable (dict-of-lists) appended to on every OnBuildInitialized with no dedup → duplicate keys pile up.

Scope

• Make event subscriptions idempotent (subscribe-once, or unsubscribe in finally).
• Clear/reset InMemorySink per run (or call Dispose).
• Dedup or reset VerbosityMapping.Mappings per run.

Acceptance criteria

• A test that calls Execute twice in one process shows: no duplicated handlers firing, no carried-over log events, no duplicate verbosity mappings.
• No public API change.

Notes

• Non-breaking. No dependencies — do this first; it's shippable on its own.
• Size: S.