fix(ios): share URLCache across fetch transports

[peterpme]

Summary

• use one internal iOS URLCache for standard and streaming fetch sessions
• preserve the existing 32 MiB memory capacity, 100 MiB disk capacity, disk path, and protocol cache policy

Why

Both iOS transports currently create separate URLCache objects backed by the same nitrofetch_urlcache disk path. Sharing one cache keeps memory and disk cache ownership consistent and makes lifecycle behavior unambiguous without disabling or clearing caching.

Closes #156

Validation

• xcrun swiftc -parse for all changed Swift files
• bun lint (passes with two existing no-shadow warnings)
• bun typecheck
• bun test --maxWorkers=2
• commit hooks

[vercel]

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