refactor(ios): define snapshot acquisition and presentation contracts

[thymikee]

Parent: #2188

Outcome

Establish the small, typed interface through which every iOS snapshot producer is planned and
published. This issue adds vocabulary and pure planning only; it must not change live producer
selection, presentation, fallback, or public command behavior.

Exact scope

Put pure vocabulary and plan models in @agent-device/contracts and pure planning/capability policy
in @agent-device/capture-kit:

• IosSnapshotInput as a closed stage union:
  • stage: 'acquired' carries an IosSnapshotAcquisition;
  • stage: 'presented' carries runner presentation plus independent validation facts.
• IosAcquisitionIntent = 'full' | 'surface-observation' as internal capture identity.
• IosSnapshotRequest, projection/presentation key, viewport evidence, and comparison identity.
• One pure request-to-CaptureHint derivation and an implementation-independent derivation table.
• Producer capabilities for acquisition depth, scope completeness, interactive-query completeness,
  viewport evidence, and available hittability evidence.
• Typed acquisition residue for provider pruning, missing viewport, truncation, stale generation,
  unavailable facts, and fallback source.
• Runner payload/qualityPayload facts sufficient for later host validation without publishing the
  quality payload.
• The IosSnapshotEngine interface with only plan(request, producer) and
  publish(input, request); implementations land in iOS snapshot backend convergence and fast Simulator observation #2188's engine child, not here.

Keep only vocabulary in contracts. Do not place rectangle folding, presentation algorithms,
semantic compaction, provider adaptation, lifecycle, fallback, or backend-name policy there.

Invariants

• Acquisition intent is part of capture/comparison identity but is not a public snapshot option.
• A producer may narrow acquisition only when its declared capability proves the narrowing complete.
• Missing facts remain missing; rectangle plus enabled never manufactures hittability.
• stage: 'presented' is accepted only from the Apple runner producer.
• Internal stage, hint, lineage, generation, residue, and quality contracts do not become required
  daemon RPC fields.
• No compatibility/fallback path is added for unreleased shapes; migrate callers when implementation
  issues land.

Non-goals

• No Swift or TypeScript geometric presenter implementation.
• No semantic-compactor move and no ADR 0004 ownership amendment; the engine issue owns that change
  once the implementation exists.
• No provider or Simulator adapter migration.
• No live routing, runner command, daemon response, CLI help, or physical-device change.

Acceptance and evidence

• Exhaustive type tests make double presentation and skipped presentation unrepresentable at the
  engine interface.
• Golden cases pin every request-to-CaptureHint derivation, including scope, depth, raw,
  interactive-only, and acquisition intent.
• Capability completeness tests pair each positive narrowing case with the closest negative case.
• Comparison identity tests reject producer, intent, generation, presentation-key, and relevant
  residue mismatches.
• A planted structural violation proves algorithms or provider/lifecycle imports cannot enter the
  contracts package and names the invariant before returning green.
• Focused tests and pnpm check:affected --run pass; report any wire-compat selection even though no
  intentional wire change is permitted.

Worker stop conditions

• Stop if the proposed interface requires a public backend flag or daemon RPC version bump.
• Stop if a type depends on a concrete Appium, Limrun, bridge, or XCTest implementation name beyond
  the closed producer vocabulary.
• Do not add a new workspace package or a generated cross-language schema in this issue.