refactor: separate feed retrieval from parsing

[chhoumann]

Summary

• separate feed-document retrieval from pure XML parsing
• keep the current Obsidian requestUrl source explicitly labeled as a legacy compatibility path
• preserve feed, episode, and GUID provenance in transient parser models so future capability issuance cannot confuse subscription, site, artwork, stream, item-link, or chapter targets
• isolate existing feed-artwork and missing-item-link fallbacks inside the legacy FeedParser projection
• preserve the matching cached-feed path and its single-request behavior
• reject parser-generated malformed XML without rejecting valid feed extension elements named parsererror

This does not claim that requestUrl is pinned or capability-secured. It remains the current behavior-compatible source until a platform adapter can satisfy the shipped DNS, peer, TLS, redirect, cancellation, and close contract.

Verification

• npm run lint
• npm run format:check
• npm run typecheck
• npm run build
• npm run test - 88 files, 1,458 tests
• npm run check:a11y - 0 errors and 0 warnings
• uv run --with-requirements docs/requirements.txt mkdocs build -f docs/mkdocs.yml -d site
• focused parser suite - 2 files, 34 tests
• three independent read-only hostile reviews - no remaining P1 or P2 findings

Real Obsidian proof

Verified in Obsidian 1.12.7 using the isolated worktree vault:

• loaded a localhost RSS fixture through the real Podcast view
• accepted a valid feed containing a parsererror extension and cached the expected episode with its legacy URL fallback and parsed 90-second duration
• reproduced Chromium's malformed-XML recovery shape, updated the detector to recognize its direct-root XHTML parser error, and verified the malformed feed was fetched but never cached
• captured no Obsidian runtime errors
• cleared the isolated settings/cache and stopped the private Obsidian process plus fixture server

Release and migration impact

• no persisted-schema change
• no user-visible behavior change except explicit malformed-XML rejection
• no release version expected from this refactor: commit

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