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/lemmas/LJ对势积分标度/stmt.typ

@@ -0,0 +1,9 @@
+设分子间对势为 Lennard–Jones 6-12 形式
+
+$ u(r) = 4 epsilon [(d \/ r)^(12) - (d \/ r)^6] . $
+
+把一个表面分子与下半空间所有分子的相互作用积分求和，得到
+
+$ sigma tilde.op epsilon / d^2 . $ <LJ标度>
+
+此式与量纲估计 @量纲估计 同标度。