feat(checker): cph-model — structured export-target build config (WU-B, ADR-0009)

manifest.toml [targets.*] upgraded from bare names to structured build configs.
Lesson.targets: Vec<String> → Vec<TargetConfig> { name, artifact, numbering }.
ArtifactKind { SingleFile, FileTree } mirrors Spec.Courseware.Artifact.
NumberingConfig.heading: Option<Vec<String>> — None means "use framework
default" (render package owns it), so absence is distinguishable from a value.
Empty [targets.x] body = all defaults, no diagnostics. Malformed config (bad
artifact value, non-array numbering.heading) → non-fatal SchemaViolation, target
kept with defaults. Enabled toml `preserve_order` so target declaration order is
real (the old Vec<String> never actually preserved order). target_names() helper
for call sites that only want names. 8 tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/cph-model/Cargo.toml

@@ -7,4 +7,6 @@ license.workspace = true
 [dependencies]
 cph-diag = { workspace = true }
 serde = { workspace = true }
-toml = { workspace = true }
+# `preserve_order` keeps `[targets.*]` tables in TOML document order, which is
+# the declared order this crate exposes via `Lesson.targets` (ADR-0009).
+toml = { workspace = true, features = ["preserve_order"] }