fix(ios): preserve passkey bytes and own request completion safely

[OskarEichler]

Fixes and behavior changes

This fixes several related iOS bridge correctness and lifetime problems:

• Preserve large-blob bytes exactly, using numeric byte indices rather than lexicographic ordering and machine-sized Int/UInt memory. A 12-byte write previously became 96 bytes on a 64-bit platform; reading returned one machine-sized integer and discarded the remainder.
• Return large-blob records with zero-based byte indices. This is an observable output correction. Previously stored blobs may contain the old corrupted encoding; inspect/rewrite them from original data rather than assuming automatic migration.
• Normalize binary extension inputs before iOS JSON serialization, so ArrayBuffer data is not lost. Share safe decoding for base64url strings, byte arrays and legacy numeric-key records; invalid indices/bytes reject rather than trap.
• Decode PRF credential keys from base64url (not UTF-8), accept both record and legacy array forms, and reject per-credential PRF when allowCredentials is absent as well as empty. Remove logging of PRF input values.
• Decode and cache valid descriptor IDs once, reject missing/invalid IDs and missing RP IDs through decoding errors, and preserve all supported transports rather than only the first.
• Stop after missing attestation/signature errors instead of force-unwrapping nil.
• Break the module/delegate retain cycle; clear handlers after settlement; make delegate completion one-shot. Reject an overlapping create/get with RequestFailed instead of overwriting the first promise. Run bridge methods on the main queue, aligned with authorization callbacks.

Public method signatures are unchanged. Invalid inputs, overlapping requests, large-blob output and PRF credential matching intentionally receive corrected behavior. A passkey creation/get ceremony is not started by any verification harness.

Verification

• Baseline and combined fixes: all 30 existing JS tests pass; TypeScript, full ESLint, CommonJS/ESM/declaration builds and diff checks pass. No test files changed.
• 2,580 JS serialization cases across both platforms and five binary representations; 387 Swift binary round trips; nine malformed input cases; multi-transport and both PRF credential-map forms.
• Swift harnesses run actual decoder/method bodies with controlled inputs: retained cycle true -> false, duplicate completions 2 -> 1, overlap no longer replaces the original handler, handlers clear on success/error. The harness is not a live native authentication session.
• Consumer verification uses a 3.6.1 backport excluding unrelated changes on stable: both Android Debug flavors, both iOS simulator Debug schemes, and all four release-mode Metro bundles pass on RN 0.87.1. Installed distributables/native sources match the verified build. App immutable install and lint pass.
• Existing Swift unreachable-default and builder-bob tsc-discovery warnings remain. No real passkey registration, login, deletion, credential Signal API call, or physical authenticator test was performed.