Fix invalid EPUB spine references

[Ray0907]

Summary

• Treat EPUB spine entries with missing idref values or dangling manifest references as failed chapters.
• Return ConvertError::Malformed when every declared spine entry is unusable.
• Preserve partial recovery when at least one spine chapter remains readable.
• Add a regression test for a spine entry whose idref is absent from the manifest.

Root cause

Spine entries were collected through filter_map, so malformed or unresolved entries were
silently discarded before the parser's failed-chapter accounting. An EPUB containing only a
dangling spine reference therefore returned an empty successful document instead of a malformed
input error.

Validation

• cargo test formats::epub::tests::unresolved_spine_itemref_is_malformed -- --exact --nocapture
• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --locked

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}