docs: fix README diagram links

[dajiaohuang]

What type of PR is this?

docs

Check the PR title.

• This PR title matches the format: <type>(optional scope): <description>.
• The description of this PR title is user-oriented and clear enough for others to understand.
• No user-documentation PR is required; this change repairs repository README assets only.

(Optional) Translate the PR title into Chinese.

docs: 修复 README 架构图链接

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

Both README variants still reference two JPEGs that were deleted when the current PNG diagrams were added. This updates the four references to tracked replacement assets:

• the project structure and modules diagram for the introductory concept section;
• the architecture overview diagram for the framework structure section.

Validation:

• verified every local Markdown image target in both README files exists;
• verified both deleted JPEG references are absent;
• git diff --check passes;
• confirmed the old raw-content URLs return 404 and the replacement URLs return 200.

zh(optional): 两份 README 仍引用已删除的 JPEG。本 PR 将 4 处链接更新为仓库中现有的项目结构图和架构总览图。

(Optional) Which issue(s) this PR fixes:

Fixes #1208

(optional) The PR that updates user documentation:

Not required; this only fixes repository README image references.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.40%. Comparing base (ebd616c) to head (6b38d5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1210      +/-   ##
==========================================
- Coverage   83.46%   83.40%   -0.07%     
==========================================
  Files         162      162              
  Lines       23676    23676              
==========================================
- Hits        19762    19747      -15     
- Misses       2644     2654      +10     
- Partials     1270     1275       +5     

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