Strip top-level Ion annotation in KFX parse_entity_ion

[Imaclean74]

Summary

Some KFX containers tag entities with an Ion type annotation, e.g.
$490::{ ... }, where the outer Ion value is an Annotated wrapper around
the entity struct. Every callsite of parse_entity_ion in import/kfx.rs
then reaches for the struct via .as_struct() (directly or via get_field),
which returns None for an Annotated value — silently dropping the
entity's fields and producing a book with empty metadata.

We've observed this on multiple real-world KFX-ZIP packages: the inner
.azw.md metadata sidecar carries a $490::{ ... }-wrapped book_metadata
entity, so title, authors, publisher, language, identifier, etc.
all return their defaults until the wrapper is stripped. A synthetic
fixture reproducing the issue is included.

This patch strips the wrapper once at the parse boundary so all importer
paths can treat the returned value as the entity itself, consistent with how
bin/kfx-dump.rs already unwraps annotated values at every consumer (~17
manual unwraps in that binary).

Changes

• src/import/kfx.rs: parse_entity_ion now matches on the parsed value
  and returns the inner value when it is an IonValue::Annotated. Existing
  behaviour for un-annotated values is unchanged.

The fix uses a move (not the existing unwrap_annotated() accessor) to
avoid a clone on the hot import path; unwrap_annotated() returns a
reference and would force the caller to clone.

Test

• tests/fixtures/annotated_metadata.kfx.gz (501 bytes) — a 641-byte
  synthetic KFX container with a single \$490::{ ... }-annotated
  book_metadata entity carrying eight synthetic categorised metadata
  fields. Built by examples/build_annotated_kfx_fixture.rs using only
  the public kfx::serialization helpers — no real-book provenance.
• tests/kfx_annotated_metadata.rs — opens the fixture via the public
  Book::open API and asserts metadata().title, authors, publisher,
  language, identifier, date all round-trip. Before this patch's fix,
  the test fails with left: \"\" / right: \"Annotated Entity Test Book\"
  on the first assertion.

Verification

cargo fmt -- --check
cargo clippy --lib --tests --examples
cargo test --lib            # 548 passed
cargo test --test kfx_annotated_metadata  # 1 passed

Reference

Amazon Ion specification on annotations — annotations are schema-level
metadata and do not change a value's logical shape:
https://amazon-ion.github.io/ion-docs/docs/spec.html#annot