Disable Default Polling for OFREP Static-Context Providers

[jonathannorris]

Problem

OFREP static-context providers currently poll on a fixed timer by default (JS: 30s, Swift: 30s, Kotlin: 5min). This has two issues:

1. Sticky bucketing — polling causes flag values to change mid-session, which breaks the expectation that a user sees consistent behavior throughout a session. Most platforms expect bucketing to remain sticky for a user's whole session.

2. Resource efficiency — timer-based polling is wasteful for static-context providers where the evaluation context rarely changes during a session. This is especially problematic on mobile where it impacts battery life and data usage.

This is out of step with how vendor SDKs handle client-side refresh. DevCycle, LaunchDarkly, Statsig, and Eppo do not use timer-based polling as the default client-side refresh mechanism:

Vendor	Default Mechanism	Timer Polling?
DevCycle	Init + context change + foreground + SSE push	No
LaunchDarkly	SSE streaming foreground, 60min poll background only	No (streams)
Statsig	Init + user change only	No
Eppo	Init only, polling opt-in	No (off by default)
ConfigCat	Auto polling (60s)	Yes
Unleash	Polling (30s)	Yes

Proposed Solution

Replace default timer-based polling with event-driven refresh for static-context providers. Providers should re-fetch bulk evaluations on:

1. Initialization (already defined in ADR-0009)
2. Context change (already handled via onContextChange)
3. App foreground / page visibility — re-fetch when the app returns from background or the page becomes visible

Timer-based polling should remain available as an opt-in for applications that want it, but default to disabled.

Further details on foreground detection, debouncing, and interaction with SSE (see #62) will follow in an ADR PR.