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/σTp态函数导言/element.toml

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

examples/TH-141/segments/σTp态函数导言/textbook.typ

@@ -0,0 +1,3 @@
+= $sigma(T, p)$ 态函数与工程应用
+
+以上所有模型都从微观或唯象规律推出 $sigma$。在工程实际中，$sigma$ 常被直接当作状态变量 $(T, p, "组分")$ 的态函数处理，由实验数据拟合给出。本节给出一道典型的工程应用题：把 $sigma(T, p)$ 当作给定的多项式态函数，用它解一个混合优化问题。