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/segments/Fowkes极性修正/textbook.typ

@@ -0,0 +1,5 @@
+Girifalco–Good 公式 @GG公式 建立在纯色散力假设上，对水这类强极性或氢键液体已不够精确。Fowkes 给出的修正路径是把表面能 $gamma$ 拆为色散分量 $gamma^d$ 与极性分量 $gamma^p$，分别做几何平均：
+
+$ W_(S L) = 2 sqrt(gamma_(S G)^d thin gamma_(L G)^d) + 2 sqrt(gamma_(S G)^p thin gamma_(L G)^p) . $
+
+工程实际计算中水、聚合物等强极性体系的接触角分析走的就是这条路。本节只列出公式作为知识地图的一站，不要求会用。