Add StreamingUxValidator for real-time UX testing

[noahgift]

Part of Epic #14

Problem

Cannot validate real-time streaming UX elements like:

• VU meters responding to audio
• Progress bars advancing
• State indicators transitioning
• Latency between events

Proposed API

pub struct StreamingUxValidator {
    states: Vec<(String, Instant)>,
}

impl StreamingUxValidator {
    /// Start tracking a state indicator element
    pub async fn track_state(page: &Page, selector: &str) -> ProbarResult<Self>;
    
    /// Assert state sequence occurred in order
    pub fn assert_state_sequence(&self, expected: &[&str]) -> ProbarResult<()>;
    
    /// Assert VU meter level changes (element width/height changes)
    pub async fn assert_level_changing(
        page: &Page,
        selector: &str,
        property: &str, // "width" or "height"
        min_change_percent: f32,
        timeout_ms: u32,
    ) -> ProbarResult<()>;
    
    /// Assert progress advances from 0 toward 100
    pub async fn assert_progress_advancing(
        page: &Page,
        selector: &str,
        timeout_ms: u32,
    ) -> ProbarResult<()>;
    
    /// Assert transition latency is within bounds
    pub fn assert_transition_latency(
        &self,
        from_state: &str,
        to_state: &str,
        max_ms: u32,
    ) -> ProbarResult<()>;
}

Use Cases

• Validate VU meter responds to audio input
• Validate state indicator shows correct states
• Validate chunk progress bar fills during recording
• Measure and assert on real-time latency

[noahgift]

Triage: P3 — WASM testing feature. Part of the multi-threaded WASM UX testing epic (#14). Not blocking core functionality.