feat(render): expose emitted CSS class names as a public constant

[P4suta]

Background

The downstream consumer afm (P4suta/afm) mirrors this renderer's
CSS-class contract by hand. In crates/afm-markdown-test-support/src/lib.rs
(lib.rs:62-86) it keeps an AFM_CLASSES: &[&str] constant and documents
why it is maintained manually:

Sibling aozora-render embeds the aozora-* class names as string
literals inside writer.write_str(...) calls — there is no exported
pub const we could import. Build-time fetch of the sibling source is
technically possible but heavyweight (depends on cargo metadata + a
regex scan).

So whenever aozora-render adds a class, afm's
tests/css_class_contract.rs fails and a human has to append the new
name to AFM_CLASSES (and to the two mdbook themes). Drift is caught,
but the fix is manual and the class list is duplicated downstream.

What to do

Expose the set of CSS class names that aozora-render can emit as part
of its public API. A few shapes that would work:

/// Every CSS class the HTML renderer can emit.
pub const AOZORA_CLASSES: &[&str] = &["aozora-bouten", /* … */];

or named consts the renderer itself uses at the write_str sites (so the
authoritative list and the emit sites cannot drift), or a
pub fn classes() -> &'static [&'static str].

Why it helps

• afm imports the constant directly and deletes its hand-maintained
  mirror + manual-append step entirely.
• Drift detection moves upstream — to where the classes are defined —
  instead of surfacing only as a failing downstream integration test.
• It unblocks afm's automation sketch (a build.rs/future-ADR that was
  going to fetch this source tree and regex-extract class="aozora-…"
  literals). With a pub const there is nothing to scrape.

References

• afm crates/afm-markdown-test-support/src/lib.rs:62-86 (the manual
  mirror + the rationale quoted above)
• afm crates/afm-markdown/tests/css_class_contract.rs (the drift gate)
• Downstream tracking issue: feat(test): automate AFM_CLASSES discovery afm#74 (afm: feat(test): automate AFM_CLASSES discovery)