Implements the four building blocks Phase 3 needs from Stream A:

- calculate_poll_delay.dart: pure helper. Given freshness and
  interval, returns the time remaining in the interval; zero when
  overdue; full interval when there's no prior freshness or the
  freshness is in the future (clock skew clamp).
- polling_initializer.dart: one-shot Initializer. Calls the injected
  PollFunction up to 3 times. ChangeSetResult and terminal status
  results return immediately. Interrupted results retry after a 1s
  delay. After 3 interrupted attempts the last is escalated to
  terminalError so the orchestrator stops retrying at this layer.
  close() interrupts a pending retry delay and yields shutdown.
- polling_synchronizer.dart: long-lived Synchronizer. Single-
  subscription StreamController that polls immediately on subscribe,
  then schedules subsequent polls via calculatePollDelay over the
  freshness of the most recent successful result. Interrupted
  results pass through but do not advance freshness, so a transient
  failure does not delay the catch-up poll. Injected TimerFactory
  and now() for deterministic tests.
- cache_initializer.dart: Initializer that reads the persistence
  cache via an injected CachedFlagsReader. Cache hit emits a full
  ChangeSetResult with persist=false and an empty selector (the
  cache does not track server-side selector state). Cache miss or
  reader exception emits a none-type ChangeSetResult so the
  initializer chain advances. The reader typedef leaves the actual
  persistence wiring to the orchestrator (Phase C1).

PollFunction, DelayFunction, TimerFactory, and CachedFlagsReader are
typedefs rather than concrete dependencies so the orchestrator can
wire real implementations and tests can inject scripted ones,
mirroring the abstraction style established in SDK-2183.

…tion to drive status reporting