Build a new Map whenever fetcher state needs to change instead of
mutating the existing map in-place.  The existing map may already have
 setState) and
React renders asynchronously, so mutating it after the fact is unsafe.

Key changes:
- updateState: delete idle mountedFetchers only AFTER subscribers are
  notified (so subscribers can capture idle fetcher data), not before
- HandleLoadersResult now carries an optional fetchers map so that
  abortStaleFetchLoads / markFetchRedirectsDone results are committed
 completeNavigation
- getUpdatedRevalidatingFetchers: build a copy, don't mutate state.fetchers
- handleFetcherAction: build updatedFetchers / finalFetchers maps instead
  of mutating state.fetchers; redirect paths advance state.fetchers via
  state = { ...state, fetchers: withDoneFetcher() }
- updateFetcherState: build new Map before calling updateState
- setFetcherError: build new Map before calling updateState
- deleteFetcher: no longer deletes from state.fetchers; callers that need
  a React-facing snapshot must build their own Map and exclude the key
- markFetchersDone / markFetchRedirectsDone / abortStaleFetchLoads: accept
  a fetchers Map parameter and operate on that instead of state.fetchers
- processLoaderData: accepts workingFetchers Map parameter

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Since state.fetchers is now always replaced with a new Map (never
mutated in place), we can key useMemo on that reference. The expensive
Array.from + spread only runs when fetchers actually change, not on
every unrelated navigation or state update.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The original test checked subscriber states and could never  thefail
subscriber sees state after each updateState call, not the intermediate
mutation. The real bug is that state.fetchers.set(key, doneFetcher)
mutates the Map reference already handed to React's concurrent renderer.

The new test captures the Map reference from the subscriber when the
fetcher enters the 'loading' state, then asserts that reference was NOT
mutated to 'idle' after the fetch completes. This fails on the dev branch
(before the fix) with 'Expected: loading, Received: idle' and passes with
the immutability refactor in place.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Adds four additional regression tests alongside the existing action+loader
test, covering every code path that previously mutated state.fetchers in
place after handing the Map reference to React:

- Fetcher GET load: updateFetcherState() called twice (loading then done)
  mutated the loading Map via the second state.fetchers.set() call.

- Fetcher revalidation during navigation: processLoaderData() mutated the
  loading Map (handed to subscribers during getUpdatedRevalidatingFetchers)
  when marking revalidating fetchers done after loaders completed.

- Fetcher loader redirect: markFetchersDone() inside completeNavigation
  mutated the loading Map handed to subscribers before the redirect finished.

- Fetcher action redirect: markFetchersDone() inside completeNavigation
  mutated the loading Map (post-action-redirect) handed to subscribers
  before the redirect navigation completed.

Each test captures the Map reference from the subscriber at the relevant
intermediate state, then asserts after completion that the Map still
reflects that intermediate state (not the final idle state). All five
tests fail on the dev branch and pass with the immutability refactor.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

React's useId() can produce IDs like _r_11_ (digits only, no trailing
letter), but the regex was /^_r_[0-9]?[a-z]_/ which required exactly
one alpha character. Broaden to /^_r_[0-9a-z]+_/ to match any
alphanumeric suffix.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>