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

## 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**.