Remove NYM mixFetch per-host request queue

[bobbythelobster]

The per-host request queue was a workaround for a mixFetch limitation that has been resolved in @nymproject/mix-fetch@1.4.2. Callers now use initMixFetch directly to obtain the fetch function.

Changes

• src/util/nym.ts: Remove queueMixFetch, hostRequestChains Map, and getHostKey helper. initMixFetch and mixFetchOptions remain unchanged.
• src/io/browser/browser-io.ts: Replace queueMixFetch call with direct initMixFetch + nymFetch call.
• src/io/react-native/react-native-worker.ts: Same as browser-io.

Impact

• 3 files changed, 8 insertions, 66 deletions (net -58 lines)
• All 162 tests pass
• TypeScript compiles cleanly

Relates to Asana task 1213688999708248

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[samholmes]

Remove this changelog entry because it's a moot point.

[bobbythelobster]

✅ Review feedback addressed:

• Removed CHANGELOG entry (commit 0d3eea7)

The CHANGELOG remains in 'Unreleased' state with no entries since the mixFetch upgrade is part of the ongoing NYM/privacy-fetch feature work, not a user-facing release change.