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/σT建模导言/element.toml

@@ -0,0 +1 @@
+kind = "segment"

examples/TH-141/segments/σT建模导言/textbook.typ

@@ -0,0 +1,3 @@
+= $sigma(T)$ 的建模
+
+以上讨论都默认温度固定。但 $sigma$ 随温度变化的图像是物理学的常识——温度升高时分子热运动加剧，表面层的亏损能下降，$sigma$ 随之下降，在临界点处趋零。本节给出 $sigma(T)$ 的几条关系：从缺键模型直接读出的 $L_m(T)$ 单调下降图像、唯象的 Eötvös 规则及其幂律修正、以及把 $sigma$ 升级为自由能的热力学关系 $dif sigma \/ dif T = - s_s$。