feat(ios): productionize the selected Simulator snapshot bridge

[thymikee]

Parent: #2188

Blocked by: #2192

Start condition: #2192 must close with an explicit GO naming the selected mechanism. A NO-GO closes
this issue without implementation; do not substitute another private mechanism ad hoc.

Outcome

Productionize the selected Simulator accessibility reader as private, host-local infrastructure
behind one @agent-device/platform-apple acquisition adapter. The bridge must build, cache, spawn,
bind, read, cancel, fail, and recover deterministically, but no production snapshot request routes
to it yet.

Exact scope

Native bridge

• Put the small native source under apple/snapshot-bridge/.
• Implement the spike-proven framed batch request/response protocol with logs on stderr.
• Return acquisition facts only: raw nodes, viewport evidence, target generation, truncation, typed
  residue, and protocol/source versions.
• Enforce request/response bytes, node count, depth, work, and duration limits.
• Return typed unsupported, malformed-tree, stale-target, timeout, cancellation, crash, and transport
  failures.

Apple host adapter and lifecycle

• Put adapter, build/cache, spawn supervision, request client, and lifecycle under
  packages/platform-apple/src/snapshot-source/ behind one narrow runtime facet.
• Cache outside the npm package using at least source hash, Xcode build, macOS version, architecture,
  and Simulator-runtime compatibility inputs.
• Compile lazily or through explicit preparation; never compile in npm postinstall.
• Use atomic cache publication and reject stale/incompatible protocol/source versions.
• Key target state by UDID plus opaque target generation. Do not retain capture data or comparison
  baselines across sessions or generations.
• Preserve request cancellation and deadline signals through build, bind, and read.
• Write bridge/process diagnostics to the owning session's runner/diagnostic artifacts without
  replacing existing warnings or typed error details.

Protocol decision

Hand-maintain the small Swift/TypeScript wire carriers and pin them with round-trip fixtures. Do not
add code generation now. Revisit generation only after an incompatible protocol evolution, a second
independent wire consumer, two demonstrated optionality/enum-drift defects, or a third runtime.

Security and proxy seam

• The bridge listener/socket is private host infrastructure and is never exposed through
  agent-device proxy.
• Do not add a bridge, helper-control, or daemon-administration HTTP route.
• Bound and validate every decoded frame before allocation-heavy work.
• Do not put daemon, proxy, signing, or user credentials in source, cache keys, logs, or artifacts.

Non-goals

• No production snapshot routing, circuit breaker, XCTest fallback, runner-demand, open/relaunch,
  provider, or proxy behavior change.
• No public CLI flag, new workspace package, downloaded executable, pre-signed artifact, or
  physical-iPhone support.
• Do not copy Swift presentation into the bridge; it acquires only.

Acceptance and evidence

• The production adapter reproduces perf(ios): decide Simulator AX bridge viability with a bounded spike #2192's fidelity and bounded-resource results through its public
  runtime facet.
• Build cache cold, hit, invalidation, concurrent publication, corrupt entry, and toolchain-change
  cases are tested.
• Spawn/read/cancel/crash/restart/stale-generation tests use typed outcomes and leave no orphaned
  helper ownership.
• A planted route/ownership violation proves the bridge cannot be reached from the proxy or outside
  the Apple runtime facet.
• Packed, installed, and bundled size deltas are reported against test(ios): establish snapshot convergence baselines and permanent evidence #2189 and stay within the accepted
  budget; the compiled cache is excluded from the npm artifact.
• Focused native/host tests, XCTest selection when applicable, layering, packaging, and
  pnpm check:affected --run pass; attach live Simulator evidence.

Worker stop conditions

• Stop if production behavior must route to the bridge to test the adapter; routing belongs later.
• Stop if cache correctness requires an unbounded compatibility fallback.
• Stop if the selected mechanism no longer meets perf(ios): decide Simulator AX bridge viability with a bounded spike #2192's GO evidence on the supported toolchain.