docs: add sources or mark illustrative for Real-World examples (#73)

[PerryLink]

Summary

Addresses #73. For each "Real-World Example"-style section in lectures 03–05 (English and Chinese), I searched for a verifiable source behind the concrete numbers and found none. Rather than fabricate links, I renamed the sections to Illustrative Example and added a one-line disclaimer stating that the scenario is a teaching illustration, not a published case study.

Changes by file

English (docs/en/lectures/)

• lecture-03-why-the-repository-must-become-the-system-of-record/index.md
  • Heading ## A Real Transformation Story → ## Illustrative Example
  • Added disclaimer: "This example is a teaching illustration, not a published case study — the numbers are illustrative rather than measured from a real project."
• lecture-04-why-one-giant-instruction-file-fails/index.md
  • Heading ## Real-World Example → ## Illustrative Example
  • Added the same disclaimer.
• lecture-05-why-long-running-tasks-lose-continuity/index.md
  • Heading ## Real-World Example → ## Illustrative Example
  • Added the same disclaimer.

Chinese (docs/zh/lectures/)

• lecture-03-why-the-repository-must-become-the-system-of-record/index.md
  • Heading ## 一个真实的改造故事 → ## 示意性示例
  • Added disclaimer: "本示例为教学示意，并非已发表的真实案例——文中数字仅为让概念更具体，并非来自真实项目的实测数据。"
• lecture-04-why-one-giant-instruction-file-fails/index.md
  • Heading ## 实际案例 → ## 示意性示例
  • Added the same Chinese disclaimer.
• lecture-05-why-long-running-tasks-lose-continuity/index.md
  • Heading ## 实际案例 → ## 示意性示例
  • Added the same Chinese disclaimer.

Verification

• npm install — completed (only npm allow-scripts warnings for esbuild, unrelated to this change).
• npm run docs:build — build complete in 33.69s, no errors.