feat: add iTunes and Podcast 2.0 namespace support (Phase 6)

[bug-ops]

Summary

Add podcast extension support for iTunes and Podcast 2.0 namespaces, enabling parsing of podcast-specific metadata from RSS feeds.

New Features

iTunes Namespace (itunes:):

• Feed-level: author, owner, categories, explicit, image, keywords, type
• Episode-level: duration, episode, season, episodeType

Podcast 2.0 Namespace (podcast:):

• podcast:guid - Permanent podcast GUID
• podcast:funding - Donation/support links

New Types

// Feed-level metadata
pub struct ItunesFeedMeta {
    pub author: Option<String>,
    pub owner: Option<ItunesOwner>,
    pub categories: Vec<ItunesCategory>,
    pub explicit: Option<bool>,
    pub image: Option<String>,
    pub keywords: Vec<String>,
    pub podcast_type: Option<String>,
}

// Episode-level metadata
pub struct ItunesEntryMeta {
    pub duration: Option<u32>,  // seconds
    pub episode: Option<u32>,
    pub season: Option<u32>,
    pub episode_type: Option<String>,
    // ...
}

Usage

let feed = parse(&xml)?;

if let Some(itunes) = &feed.feed.itunes {
    println!("Podcast by: {}", itunes.author.as_deref().unwrap_or("Unknown"));
    println!("Explicit: {:?}", itunes.explicit);
}

for entry in &feed.entries {
    if let Some(itunes) = &entry.itunes {
        println!("Duration: {} seconds", itunes.duration.unwrap_or(0));
    }
}

Reviews Completed

Review	Status
Performance	✅ EXCELLENT (28-37% faster)
Security	✅ PASS (no vulnerabilities)
Testing	✅ PASS (180 tests)
Code Review	✅ APPROVED

Test plan

• All 180 tests pass
• New integration test for iTunes podcast feed
• cargo clippy --all-targets --all-features -- -D warnings passes
• cargo +nightly fmt --check passes
• cargo doc --no-deps --all-features passes

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