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/缺键模型导言/element.toml

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

examples/TH-141/segments/缺键模型导言/textbook.typ

@@ -0,0 +1,3 @@
+== 缺键模型
+
+缺键模型在 @骨架公式 的两个因子上各加一层物理输入。亏损能由"表面分子比体相分子缺多少根键"乘以"每根键的能量"给出，单键能由摩尔汽化热反推；面密度由"每个分子占的体积"开 2/3 次方给出。模型只保留一个无量纲参数 $zeta$——表面分子近邻数与体相近邻数之比。下面给出一般式，再讨论它的两个特例：$zeta = 1\/2$ 对应不带任何晶格信息的 Stefan 估算，立方格子下 $zeta$ 可以精确取到 $5\/6$、$3\/4$ 等具体数值。