Make iOS simulator interactive bitrate an optional cap, uncapped by default

[jkelleyrtp]

Summary

Tuning::Interactive { bitrate: None } no longer sets any rate-control property: the 0.15 bpp derivation (TARGET_BITS_PER_PIXEL, resolved_bitrate) is gone, and an explicit Some(bps) remains an optional AverageBitRate cap. Low-latency rate control, RealTime, AllowFrameReordering=false, MaxFrameDelayCount=0, profile and keyframe settings are unchanged.

The tuning-to-property translation is now a single seam, unit-tested:

fn rate_control(tuning: Tuning) -> RateControl {
    Interactive { bitrate: None }    => RateControl::Uncapped,        // no property set
    Interactive { bitrate: Some(b) } => RateControl::AverageBitRate(b),
    Recording { quality }            => RateControl::Quality(quality.clamp(0.0, 1.0)),
}

Stats gain an additive target_bitrate: Option<u32> (iOS StatsReport → serve-layer StatsReport → /api/stats): None means uncapped, Some an active cap. Android maps None. CLI --bitrate help and Tuning docs updated.

Measurements

1206x2622 device encoding at 588x1280, screen fully covered by CSS animation, 25 s windows, no subscribers:

config	Mbps	bpp	mean frame	fps
uncapped (low latency)	0.37–0.48	0.029–0.030	2.7 KB	16–23
6 Mbps (low latency)	5.84	0.1302	12.0 KB	59.6
24 Mbps (low latency)	8.80	0.1965	18.1 KB	59.5
uncapped, low latency OFF (experiment only, not shipped)	4.37	0.2272	20.9 KB	25.6

Session creation succeeds without AverageBitRate, but with low-latency rate control the encoder picks an internal budget under 0.5 Mbps and drops most frames staying inside it — the capped runs sustained ~60 fps on the same stimulus. Recorded in .devin/skills/ios-simulator-video/SKILL.md. This PR ships the spec as requested (uncapped default, low-latency behaviour untouched); whether the default should change in light of these numbers is left as a follow-up decision.

Checks

cargo fmt, cargo build --workspace, cargo clippy --workspace --all-targets (only pre-existing warnings), cargo test --workspace --lib, cargo test -p accessibility-cli --test cli_smoke — all green.

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/50a3e0f756984f20a1779e22d438ce96
Open in Devin Desktop: https://dioxus.staging.devinenterprise.com/desktop/session/50a3e0f756984f20a1779e22d438ce96?variant=devin-insiders
Requested by: @jkelleyrtp

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