fix: enforce strict clippy lints and eliminate technical debt

[bug-ops]

Summary

• Enforce unwrap_used, expect_used, panic clippy lints as deny at workspace level
• Configure test exemptions via cfg_attr(test, allow(...))
• Fix iTunes self-closing tag parsing bug (added is_empty parameter)
• Refactor parse_channel() from 104 to 66 lines (extracted helper functions)
• Remove all TODO comments and phase mentions from code/documentation

Changes

Clippy Lints

Added strict lints to Cargo.toml:

[workspace.lints.clippy]
unwrap_used = "deny"
expect_used = "deny"
panic = "deny"

Test Exemptions

Test files have proper allow attributes:

#![allow(clippy::unwrap_used, clippy::expect_used, clippy::panic)]

Bug Fixes

• Fixed iTunes self-closing tag parsing in parse_item_itunes()

Code Quality

• Extracted parse_channel_item() and parse_channel_extension() helpers
• Reduced parse_channel() from 104 to 66 lines
• Removed stale TODO comments and phase mentions

Test plan

• All 574 tests pass
• No clippy warnings for panic/unwrap/expect in production code
• Test code still allowed to use unwrap/expect for clarity

[codecov-commenter]

Codecov Report

❌ Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/feedparser-rs-core/src/parser/rss.rs	86.76%	9 Missing ⚠️

@@            Coverage Diff             @@
##             main      #37      +/-   ##
==========================================
+ Coverage   92.14%   92.19%   +0.05%     
==========================================
  Files          34       34              
  Lines        7446     7481      +35     
==========================================
+ Hits         6861     6897      +36     
+ Misses        585      584       -1     

Flag	Coverage Δ
rust-core	92.19% <86.76%> (+0.05%)	⬆️

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/types/generics.rs	94.82% <ø> (ø)
crates/feedparser-rs-core/src/parser/rss.rs	94.93% <86.76%> (+0.15%)	⬆️

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