labels/xml: shared tolerant XML helper, paramount + criterion onto it

Replaces two near-duplicate hand-rolled XML scrapers in paramount.rs
and criterion.rs with a single labels::xml module that's robust to:

- Case-insensitive tag / attribute names ('<Playlist>' matches the
  same as '<playlist>'; 'Name=...' matches 'name=...').
- XML namespace prefixes (matches '<ns:tag>' for tag='tag').
- Arbitrary whitespace inside open tags and around '=' separators
  ('<tag  name = "X">' works).
- Both quote styles for attribute values (" and ').
- Self-closing tag forms ('<tag/>' and '<tag />').
- '>' chars inside quoted attribute values (no premature end-of-tag).

Three functions:
  xml::attr(element, name) -> Option<String>
      Extract attribute value from an open-tag fragment.
  xml::text(xml, tag) -> Option<String>
      Trimmed text content of first <tag>...</tag>.
  xml::find_element(xml, tag, from) -> Option<(start, end)>
      Locate next <tag>...</tag> for iteration; handles self-closing.

22 unit tests cover the robustness properties: case-insensitivity,
namespace stripping, whitespace tolerance, quote styles, self-close
forms, no-substring-false-positive (looking for 'lang' must NOT
match 'lang_id' or 'language'), '>' inside quoted attrs, iteration
across repeated elements.

paramount.rs: drops local extract_attr; find_feature_playlist now
walks xml::find_element('playlist', ...) so it works regardless of
case and self-closing style. Pre-refactor: required exactly
'<playlist ' (single space, exact case) and '/>' for self-close.

criterion.rs: drops local extract_tag; parse_stream_infos and
parse_playback_config iterate via xml::find_element. Same case-
sensitivity + namespace gains. The 'COMMENTARY' / 'SDH' / 'DS'
content-value match is now case-insensitive too (previously a disc
authored with 'commentary' would have been miscategorized as Normal).

Pre-refactor known failure modes (none observed yet, but trivial
to trip on a future disc): vendor switches whitespace around '=',
uses single quotes, capitalizes a tag, prefixes a namespace. All
now handled.

Out of scope by design: XML entity decoding (&amp;, &lt;), CDATA
sections, comments, processing instructions. None observed in BD-J
authored label data. If a future disc trips them, the entity
decoder is a localized addition.

Precommit (cargo +1.86 fmt + clippy + test) green.

src/labels/mod.rs

@@ -17,6 +17,7 @@ mod paramount;
 mod pixelogic;
 pub(crate) mod text;
 pub mod vocab;
+pub(crate) mod xml;
 
 use crate::disc::{DiscTitle, Stream};
 use crate::sector::SectorReader;