feat(test): automate AFM_CLASSES discovery

[P4suta]

Background

afm pins the renderer's CSS-class contract in
crates/afm-markdown-test-support/src/lib.rs as the AFM_CLASSES
constant, maintained by hand. The file documents why (lib.rs:62-86):
aozora-render embeds the aozora-* class names as string literals
inside writer.write_str(...) — there is no pub const to import — so
each new upstream class has to be appended here (and to the two mdbook
themes) when tests/css_class_contract.rs fails after an aozora-* pin
bump.

The same file (lib.rs:82-86) sketches the intended fix as a future ADR:

A dedicated automation pass (build.rs that fetches the sibling source
tree, regex-extracts class="aozora-…" literals, and emits an
AFM_CLASSES const into OUT_DIR) is sketched as a future ADR — it
would eliminate the manual append step at the cost of a network /
filesystem dependency in the test build.

What to do

Eliminate the manual-append step. Two paths, in preference order:

1. Preferred — consume an upstream constant. If aozora-render
   exposes its emitted class names as a pub const (tracked in
   feat(render): expose emitted CSS class names as a public constant aozora#91), AFM_CLASSES becomes a direct re-export with no
   scraping and no build.rs. This is the cheap, robust outcome and makes
   this issue mostly a follow-the-upstream task.
2. Fallback — local automation. If upstream does not expose it, write
   the build.rs sketched above (fetch sibling source via cargo
   metadata, regex-extract class="aozora-…", emit into OUT_DIR), and
   record the network/filesystem-dependency trade-off in an ADR.

Either way, also keep crates/afm-book/theme/afm-{horizontal,vertical}.css
in sync — consider deriving/checking the themes' class coverage from the
same source of truth.

References

• crates/afm-markdown-test-support/src/lib.rs:62-86 (manual mirror +
  future-ADR sketch)
• crates/afm-markdown/tests/css_class_contract.rs (the drift gate)
• Upstream enabler: feat(render): expose emitted CSS class names as a public constant aozora#91
  (feat(render): expose emitted CSS class names as a public constant)
• CLAUDE.md → "CSS class contract drift"

[P4suta]

Promoting this under the umbrella #81. Implementing the pin-independent half now: a hermetic render-corpus contract test that closes the render→class→AFM_CLASSES loop (rendered classes ⊆ AFM_CLASSES; no stale entry), so the CSS area (#82) fails loudly with the exact missing class on a future pin bump instead of shipping silent unstyled drift. The heavier build.rs that scrapes aozora-render source to eliminate the manual AFM_CLASSES append (network/fs/hermeticity tradeoff) stays deferred here as the follow-up.