Add LaTeX thesis scaffold and full Chapter 1

[kumizeod]

Motivation

• Replace the previous high-level outline with a directly executable LaTeX thesis structure so the repository can be used as a compilable paper project.
• Provide the complete first chapter (绪论) so writing can proceed immediately from a finished LaTeX source instead of another abstract plan.

Description

• Rewrote 论文目录修改方案（LaTeX）.md into a concrete execution guide that maps existing repo artifacts to chapter locations and lists per-file modification steps.
• Added thesis/main.tex as a ctexbook entrypoint with essential package setup, TOC, and commented \input hooks for subsequent chapters.
• Added thesis/chapters/chapter1_intro.tex containing the full Chapter 1 (background, problem definition, goals/contributions, method/technical route, thesis structure, and chapter summary).

Testing

• Attempted to compile the thesis with cd thesis && xelatex -interaction=nonstopmode -halt-on-error main.tex, which failed because xelatex is not available in the environment (command not found).
• Verified the new and updated files are present and their contents via nl -ba/cat, and confirmed the Chapter 1 text and main.tex structure are written as intended.

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Codex Task