feat: type optimizations - semantic newtypes and boxed namespace metadata

[bug-ops]

Summary

This PR introduces Phase 1 type optimizations for improved type safety and memory efficiency:

• Semantic newtypes: Url, MimeType, Email for type-safe handling
• Boxing large optional structs: Reduces stack size for feeds without namespace metadata
• Arc for MimeType: Efficient cloning with string interning

Breaking Changes

This is a breaking change in the public API. Type changes:

• Link.href: String → Url
• Link.link_type: Option<String> → Option<MimeType>
• Person.email: Option<String> → Option<Email>
• Enclosure.url/enclosure_type: String/Option<String> → Url/Option<MimeType>
• MediaContent/MediaThumbnail: URL and MIME type fields
• Image.url: String → Url
• Podcast types: Various URL and MIME type fields
• Namespace metadata: Option<T> → Option<Box<T>>

Migration

The newtypes implement Deref<Target=str>, so most code using string methods will work unchanged. For explicit conversions:

// Creating
let url = Url::new("https://example.com");
let url: Url = "https://example.com".into();

// Extracting
let s: &str = &url;           // via Deref
let s: String = url.into_inner();

Performance

• No regression: Benchmarks show performance within noise margin
• Memory savings: ~7.6 KB per 100-entry plain RSS feed (76% reduction in namespace overhead)
• Fast cloning: MimeType uses Arc<str> for ~10x faster clone vs String

Test plan

• All 595 tests pass
• Clippy passes with no warnings
• Benchmarks show no performance regression
• Security audit: APPROVED
• Python bindings updated and tested
• Node.js bindings updated and tested

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.46789% with 59 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/feedparser-rs-core/src/types/common.rs	80.89%	47 Missing ⚠️
crates/feedparser-rs-core/src/parser/rss.rs	95.34%	6 Missing ⚠️
crates/feedparser-rs-core/src/parser/json.rs	62.50%	3 Missing ⚠️
crates/feedparser-rs-core/src/namespace/georss.rs	75.00%	2 Missing ⚠️
...es/feedparser-rs-core/src/namespace/dublin_core.rs	87.50%	1 Missing ⚠️

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
- Coverage   92.19%   91.84%   -0.36%     
==========================================
  Files          34       34              
  Lines        7481     7758     +277     
==========================================
+ Hits         6897     7125     +228     
- Misses        584      633      +49     

Flag	Coverage Δ
rust-core	91.84% <86.46%> (-0.36%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
crates/feedparser-rs-core/src/lib.rs	20.95% <ø> (ø)
crates/feedparser-rs-core/src/namespace/cc.rs	99.34% <100.00%> (ø)
crates/feedparser-rs-core/src/namespace/content.rs	100.00% <100.00%> (ø)
...ates/feedparser-rs-core/src/namespace/media_rss.rs	97.67% <100.00%> (-0.03%)	⬇️
...es/feedparser-rs-core/src/namespace/syndication.rs	99.08% <100.00%> (ø)
crates/feedparser-rs-core/src/parser/atom.rs	89.71% <100.00%> (ø)
crates/feedparser-rs-core/src/parser/rss10.rs	91.16% <100.00%> (ø)
crates/feedparser-rs-core/src/types/entry.rs	87.69% <100.00%> (ø)
crates/feedparser-rs-core/src/types/feed.rs	96.36% <100.00%> (ø)
crates/feedparser-rs-core/src/types/podcast.rs	100.00% <100.00%> (ø)
... and 5 more

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