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/Stefan极简估算/proof.typ

@@ -0,0 +1,9 @@
+在 @缺键一般式 中取 $zeta = 1\/2$，
+
+$ sigma_(L G) = 1/2 dot L_m thin rho^(2\/3) / (mu^(2\/3) N_A^(1\/3)) . $
+
+由 $d^3 = mu \/ (rho N_A)$ 得 $rho^(2\/3) \/ (mu^(2\/3) N_A^(1\/3)) = 1 \/ (d^2 N_A)$，代入即得
+
+$ sigma_(L G) thin d^2 approx L_m / (2 N_A) , $
+
+即 @Stefan估算。