Capture unknown elements into one extension tree

[mmcdole]

Fixes #276. Fixes #203. Fixes #82.

Unknown elements were handled three inconsistent ways: namespaced elements became the recursive ext.Extension tree, non-namespaced item children went into the flat Item.Custom map (dropping nesting, attributes and repetition, which is #203's corruption and #82's missing data), and unknown elements at channel level, atom feed, entry and source level were skipped and lost entirely.

This lands the #276 plan in its v1-additive form:

• shared.ParseCustom parses any unrecognized non-namespaced element with the same recursive builder namespaced extensions already use, and files it under the _custom pseudo namespace, preserving nesting, attributes and repetition. After Route the parser element loops through one shared helper #333 the capture point was one default: case per container, so the wiring is one line each in the rss channel/item and atom feed/entry/source parsers. The RSS 1.0 <items> rdf:Seq stays skipped: it is a structural list of item references, not content.
• Item.Custom stays as a compatibility shim fed from the tree. Childless elements keep their flat value exactly as before (entity-decoded, last one wins); nested elements no longer produce corrupted flat text, they live fully formed in the tree. The field is documented as deprecated in favor of the tree; removal waits for v2.
• Feed and Item gain GetExtension, GetExtensionValue and GetCustomValue accessors (the ergonomic layer from the old v2-custom-elements branch, adapted). That branch's parser approach was not used: it captured customs with flat ParseText, so it relocated the corrupted / mangled nested custom XML #203 corruption rather than fixing it.

Behavior notes: everything here is additive. Four existing fixtures gain extensions._custom blocks (82 added lines, zero removed or changed values, custom maps byte-identical). Data that was previously discarded (channel-level and atom-level unknowns) now appears in Extensions; feeds without unknown elements produce identical output.

Tests: nine new fixture pairs (channel-level customs, attributes, repetition, CDATA, nesting, custom-plus-extension mixes, atom feed and entry customs, and a nested-with-CDATA case reproducing #203) plus an accessor unit test covering namespaced lookups, feed and item custom values, repetition, nested children, the shim's childless-only contract, and absent-key behavior.

[codecov]

Codecov Report

❌ Patch coverage is 84.31373% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.82%. Comparing base (16bfe36) to head (4103507).

Files with missing lines	Patch %	Lines
extension_helpers.go	83.33%	3 Missing and 1 partial ⚠️
internal/shared/extparser.go	75.00%	1 Missing and 1 partial ⚠️
rss/parser.go	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #334      +/-   ##
==========================================
+ Coverage   79.75%   79.82%   +0.06%     
==========================================
  Files          22       23       +1     
  Lines        1912     1948      +36     
==========================================
+ Hits         1525     1555      +30     
- Misses        298      302       +4     
- Partials       89       91       +2     

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