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/微观派Lm下降/textbook.typ

@@ -0,0 +1 @@
+从缺键模型 @缺键一般式 看，$sigma$ 对 $T$ 的依赖完全由 $L_m(T)$ 携带——另几个量 $rho$、$mu$、$N_A$ 与温度关系微弱，可视为常数。摩尔汽化热 $L_m$ 随 $T$ 升高单调下降：分子热运动加剧，平均键合能减弱，把分子从液体里拉出来所需的能量变小。临界点处 $L_m arrow.r 0$，故由 @缺键一般式 有 $sigma arrow.r 0$。这一行为是微观模型内禀给出的，不依赖任何外加假设。