feat(examples): migrate TH-141 sample to declarative layout (WU-6)

Real-content fixture for the end-to-end pipeline. TH-141 (39 parts: 22 segment,
15 lemma, 2 example) migrated from the prototype's typst `#let parts` manifest
to ADR-0008: declarative manifest.toml (project+info+ordered parts+targets) +
per-element element.toml (kind + scalars; examples carry `source`). Content .typ
files copied byte-identical (no math corruption); per-element main.typ + meta.toml
dropped (wiring is now generated). Part order matches source exactly; 5 lemmas
have no proof.typ (optional); no cross-file imports / paralearn refs / figs.

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

examples/TH-141/lemmas/Dupré关系/stmt.typ

@@ -0,0 +1,5 @@
+设把单位面积固液界面剥离成各自暴露的固气面与液气面所需的可逆功为粘附功 $W_(S L)$，则
+
+$ gamma_(S L) = gamma_(S G) + gamma_(L G) - W_(S L) . $ <Dupré关系>
+
+$W_(S L)$ 越大，固液之间越"粘"得紧，$gamma_(S L)$ 越小。