v1.5.3 — Book-wide prose style pass: no colons, em-dashes, or possessives

Full editorial pass over every chapter (1–14), the appendix solutions, and the front-matter reference pages, rewriting prose to avoid colons and em-dashes (ambiguous against Lean's own : type ascription and -- line-comment syntax) and English possessive ('s) constructions. Code blocks, LaTeX math, chapter/section titles, and direct quotes from external sources are untouched. The changelog directory was left out of scope, since it is release history rather than reader-facing narrative prose.

See lean_book/changelog/v1.5.3.md for the full changelog entry.