release the streaming flag

[czxtm]

we have a streaming option behind an experimental flag, thanks to Juanpe Bolívar (@arximboldi) for this, it's definitely an improvement so lets remove the flag and release it

---

Decision: remove the experimental preference entirely (no off-switch). The provider capability check and runtime fallback already cover providers that can't stream. Agreed by Cooper (this ticket, 08-08) and Juan (Slack, 08-15).

**Where: **experimentalStreamingEvolve in apps/native/src-tauri/src/shared_types/prefs.rs (GlobalPreferences, default false), read via state/ui_prefs.rs::experimental_streaming_evolve in evolve/mod.rs:947, gated by provider.supports_streaming() at :1293 / :1431, with a runtime fallback at :1366 that disables streaming for the rest of the run if the provider doesn't actually stream. The toggle lives in components/widget/settings/developer-tab.tsx. Built by Juan in #561.

Acceptance criteria

• Streaming is on by default for every provider whose supports_streaming() returns true (currently OpenAI-compatible and Ollama; CLI provider stays non-streaming). The existing runtime fallback is kept.
• The experimentalStreamingEvolve preference and its Developer-tab toggle are removed: GlobalPreferences / GlobalPreferencesUpdate in prefs.rs, ui_prefs.rs accessor, generated IPC types (ipc/types.ts, orpc-bindings.ts), types/preferences.ts, utils/test-fixtures.ts, ai-provider-migration.test.ts.
• Existing preference files that still contain experimentalStreamingEvolve load without error (GlobalPreferences is #[serde(default)]; add a test that a prefs JSON with the stale key deserializes).
• The evolve-progress Storybook story no longer describes streaming as an experimental flag.
• Optional, separate ticket if wanted: telemetry event for "run streamed" vs "fell back to non-streaming" so we can see how often the fallback fires.

[linear-code]

ENG-619