fix(epub): read a repeated spine part once

[tomsideguide]

The spine was never deduplicated, so n itemrefs naming one href parsed it n times. In the reported file that href is the package document itself, whose size grows with n, so a 35 KB book took 27 seconds and a 69 KB one did not finish.

Itemrefs now resolve to a part path once and dedupe on it, which also catches two hrefs spelling the same part differently.

This closes a second case the issue does not cover: pointed at a real chapter instead of the package document, the same file rendered that chapter n times, emitting one anchor id n times and turning 19 KB of input into 3.2 MB of output. Duplicate anchor ids make intra-book links ambiguous.

n=6400 goes from 27s to 0.01s, n=12800 from not finishing to 0.4s, and the amplified case from 3.2 MB to 1011 bytes.

Closes #43

---

Summary by cubic

Deduplicates EPUB spine entries by resolved part path so a repeated part is parsed and rendered once. Previously each repeated itemref re-parsed and re-rendered the same part (often the package document), duplicating anchors and causing severe slowdowns; now each unique part is processed once in first-occurrence order.

• Resolves each itemref to a path and de-duplicates entries that resolve to the same part (including differently spelled hrefs), preserving first occurrence order.
• Error handling: tracks total spine entries and converted chapters; returns an error only if none convert; warns on unresolvable or unusable chapters, and now includes the resolve error detail in the warning.
• Adds a test asserting 64 repeated itemrefs yield a single chapter’s text; performance improves from 27s→0.01s at n=6400 and from hang→0.4s at n=12800; amplified output drops from 3.2 MB to 1011 bytes.
• Closes (Optimization?) A 35 KB file is taking ~30s because each itemref re-parsing its part #43.

^{Written for commit 903e009. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 1 file

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>