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/固液界面导言/element.toml

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

examples/TH-141/segments/固液界面导言/textbook.typ

@@ -0,0 +1,3 @@
+= 固液界面 $sigma_(S L)$ 的微观建模
+
+固液界面的能量由两件事的净结果决定：固体表面原子失去固–固近邻、同时获得固–液异种近邻。前者可继续沿用缺键模型，后者引入新的物理——异种相互作用强度怎么估。本节先用 Dupré 思想实验把 $gamma_(S L)$ 改写为粘附功 $W_(S L)$，再用 Berthelot 几何平均估计 $W_(S L)$，最终给出 Girifalco–Good 公式。