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/不同物质γ量级对比/textbook.typ

@@ -0,0 +1,16 @@
+不同物质的表面张力系数差异可达两个量级。
+
+#figure(
+  table(
+    columns: (auto, auto),
+    align: (left, left),
+    table.header[*物质*][*$gamma$ ($"J/m"^2$)*],
+    [水（液气界面）], [$approx 0.072$],
+    [分子晶体], [$0.05 thin "—" thin 0.1$],
+    [离子晶体], [$0.2 thin "—" thin 1$],
+    [金属（Au、Fe、W）], [$1 thin "—" thin 3$],
+  ),
+  caption: [常见物质表面张力系数的典型量级]
+) <γ量级表>
+
+该量级谱可由缺键模型 @固气缺键式 解释。分子晶体的键合主要是范德瓦尔斯力，$L_s$ 小、$gamma$ 小；离子晶体的键合包含库仑作用，$L_s$ 上升一个量级，$gamma$ 也随之上升；金属内部的金属键键合最强，$L_s$ 最大、$gamma$ 最大。同一公式横跨四个数量级的物理体系都给出对的量级。