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/LJ积分导言/element.toml

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

examples/TH-141/segments/LJ积分导言/textbook.typ

@@ -0,0 +1,3 @@
+== Lennard–Jones 对势积分
+
+缺键模型把分子间作用力简化为最近邻间存在的离散键。若代之以连续对势函数 $u(r)$、对表面分子与下半空间所有分子之间的相互作用直接积分，得到的标度律应当与缺键模型一致，并与最初的量纲分析 @量纲估计 互为印证。本节以 Lennard–Jones 6-12 势为例完成这一积分。