fix(webrtc): give weak links room to back off instead of wedging

[gwdp]

Why

minBitrateBps was 90% of the target, so bandwidth estimation had nowhere to back off to on a path slower than the target. The encoder kept overshooting and the stream wedged.

maintainFramerate holds frame rate by dropping resolution. A 1206-wide surface holds 60fps at 300x654, where UI text is unreadable.

How

• Floor the minimum at max(300 kbps, target / 10), never above the target
• degradationPreference to balanced

Test Plan

• CI
• Shaped link with dnctl: loss 58% -> 1.5%, rtt 420ms -> 125ms
• Five concurrent viewers on an EAS worker: traded between full resolution at 30fps and 1748 at 58fps instead of collapsing resolution
• All live testing ran against a build of the stack tip (feat(webrtc): raise the offer's H.264 level to keep native resolution #171), which carries all five PRs, not this branch alone

[linear-code]

ENG-26485