feat(rss2): allow explicit override of guid isPermaLink attribute

[mmilanovic4]

Closes #191

Problem

The isPermaLink attribute on the RSS 2.0 <guid> element is currently inferred purely from which fields are set on an item, with no way to override it:

if (entry.guid) item.guid = { _text: entry.guid, _attributes: { isPermaLink: false } };
else if (entry.id) item.guid = { _text: entry.id, _attributes: { isPermaLink: false } };
else if (entry.link) item.guid = { _text: sanitizeUrl(entry.link), _attributes: { isPermaLink: true } };

This breaks down in two real cases:

• id/link is a genuine, permanent URL, but setting id currently forces isPermaLink="false" anyway.
• id is an opaque, non-URL identifier (e.g. a UUID) and a link is also present — there's currently no way to force isPermaLink="false" without dropping link entirely, which is the case raised in Way to set guid isPermaLink attribute #191.

Solution

Adds an optional isPermaLink?: boolean field on Item. When set, it takes precedence over the existing inference:

feed.addItem({
  title: post.title,
  id: post.url,
  link: post.url,
  isPermaLink: true, // guid is a real permalink, force isPermaLink="true"
  // ...
});

When omitted, behavior is fully unchanged — this is a non-breaking, additive change.

Testing

Added 4 new tests in an isolated describe block, using createSampleFeed() rather than the shared, accumulating sampleFeed fixture (so they don't shift snapshot output for other tests):

• explicit isPermaLink: true with id/link set → isPermaLink="true"
• explicit isPermaLink: false with only link set → isPermaLink="false"
• no isPermaLink + guid set → still defaults to isPermaLink="false" (no regression)
• no isPermaLink + only link set → still defaults to isPermaLink="true" (no regression)

pnpm build, pnpm lint, and pnpm test all pass locally (32/32 tests, no snapshot changes to existing tests).

Related

• Way to set guid isPermaLink attribute #191
• Implement "isPermaLink" for item:guid #89 (original attempt at exposing isPermaLink)
• For rss2, do not mark guid as permalink, unless only link is given #213 (introduced the current inference-only default behavior in 5.1.0)

Closes #191