feat: add Network migration infrastructure and opt-in backend

[rchatham]

Summary

Turn the initial Network.framework prototype into a safely selectable backend with the migration infrastructure required by later work.

Changes

• Add the Network migration CI baseline and framework-neutral compile/test coverage.
• Serialize observable and multi-observable mutation while preserving synchronous delivery semantics.
• Add async observable scaffolding.
• Add Network listener, browser, advertiser, and session transport adapters.
• Add bounded Bonjour discovery metadata with UTF-8 byte-safe TXT records and local-self filtering.
• Add the public opt-in backend selector while preserving MultipeerConnectivity as the default.
• Add public optional availableMultipeerSession feature detection while retaining the existing multipeerSession API.
• Remove unused Network coordinator advertiser-observer plumbing.
• Incorporate secure peer-identity archiving, handshake-error propagation, and efficient frame decoding from the updated base.

Compatibility

Existing initializers and Multipeer-backed behavior remain source-compatible. Apps must explicitly choose .networkFramework; MultipeerConnectivity remains the default and fallback path. availableMultipeerSession allows callers to feature-detect the MC-specific session without triggering the existing non-optional accessor's unsupported-backend precondition.

Security and discovery

Discovery TXT entries are bounded by encoded UTF-8 byte length, reject malformed oversized records, and preserve valid scalar boundaries. Discovery metadata remains self-asserted and is not an authorization identity.

Verification

• swift test — 140 tests passed locally.
• iPhone 16 / iOS 18.3.1 Xcode tests — 140 tests passed locally.
• Added backend selection, optional-session, identity, handshake-failure, frame-decoder, TXT-boundary, multibyte, and truncated-self-filter coverage.
• Correctness and security reviews found no blockers.

Stack

• Parent: feat: introduce Network framework transport prototype #22
• Child: feat: secure, harden, and verify the Network backend #42
• Consolidates the former PRs Add Network migration CI baseline #23, Serialize event observer mutations #24, Add async observable scaffolding #25, Add Network transport adapters #27, and Add Network discovery metadata #28; their commits and branches remain preserved.

[rchatham]

GitHub prevents changing the base of a pull request that is registered as part of a stack. Closing this retained checkpoint so it can be recreated with the consolidated base; the branch, commits, review history, and this PR remain preserved.

[rchatham]

Replaced by consolidated PR #57: #57