feat: always-visible mic level meter in the header

[joeblau]

What

Replaces the redundant "Stream" title with a live microphone meter — a mic glyph plus a five-segment ladder — so the answer to "is my mic working?" is on screen at all times instead of buried in Settings → Audio.

Three states are distinguishable at a glance:

What you see	What it means
Bars moving	Audio is reaching the encoder
Amber glyph, dark bars	No meter samples at all — the mic isn't feeding the stream
Red mic.slash	Muted (gain 0)

How

While broadcasting the level costs nothing: it's read off the existing MicrophoneLevelChannel that ScreenCaptureController already publishes at 10 Hz, post-gain. When idle the monitor opens its own AVAudioEngine tap so the mic can be verified before going live.

MicrophoneLevelMonitor moves out of SettingsView into AudioInputProvider (which owns AudioSessionCoordinator, its dependency) and is now owned by ContentView as a single shared instance — two instances would open competing record sessions on the local-capture path. Since the toolbar meter and the Audio settings pane can be on screen together, start/stop is reference-counted through a Client token, so closing the sheet mid-broadcast can't silence the meter behind it.

Going live stays safe: capture and the meter share the serial AudioSessionCoordinator, so the idle tap's teardown remains ordered ahead of ScreenCaptureKit's session activation.

Behavior changes worth reviewing

• The orange mic indicator is on whenever Stream is foregrounded. Inherent to metering an idle mic. It's released as soon as the app leaves the foreground, so the mic is never held behind other apps.
• Mic permission is now requested at launch (previously deferred to Settings). An always-on meter that can only read "no signal" until the user happens to open Settings is a dead end.

Also: button latency

Haptics.tap() allocated a fresh UIImpactFeedbackGenerator on every press with no prepare(), cold-starting the Taptic Engine inside each button's main-actor action — felt as the whole control responding late, and every button in the app routes through it. Now one long-lived generator, re-armed after each tap and warmed at scene activation.

Scope note

This PR also carries two threads of prior uncommitted working-tree work that couldn't be cleanly separated from the above (they overlap inside ContentView.swift):

• Debounced, off-main-actor settings persistence (SettingsPersistenceCoordinator / SettingsWriter, SettingsStore.saveConnection + saveNonSecret)
• WHIP H.264-only codec gating (StreamProtocol.supportedVideoCodecs, effectiveVideoCodec) with its StreamCore tests

Verification

• Builds against the iOS 27 SDK for device (-sdk iphoneos27.0)
• The 157-test StreamCore suite passes on an iOS 26 simulator — the same suite CI runs
• Not verified: the meter's on-device appearance. iOS 26+ toolbar glass grouping and the 3.5×14pt segment sizing were chosen by eye, not seen on hardware.

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