feat: add weekly contest 509

[yanglbme]

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Pull request overview

Adds indexing and documentation for Weekly Contest 509 problems, and introduces solution implementations for some of the newly listed problems (plus supplemental solution-file additions for Biweekly Contest 186 items).

Changes:

• Update solution index tables (CN/EN) and contest listings (CN/EN) to include Weekly Contest 509 (3982–3985).
• Add full multi-language solutions and writeups for 3982 (including TypeScript).
• Add solution source files + expanded writeups for 3978 and 3979 (Biweekly Contest 186).

Reviewed changes

Copilot reviewed 31 out of 33 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
solution/README.md	Adds Weekly Contest 509 entries to the main CN solution index.
solution/README_EN.md	Adds Weekly Contest 509 entries to the main EN solution index.
solution/CONTEST_README.md	Adds Weekly Contest 509 section (CN) with links to 3982–3985.
solution/CONTEST_README_EN.md	Adds Weekly Contest 509 section (EN) with links to 3982–3985.
solution/3900-3999/3985.Palindromic Subarray Sum/README.md	Adds CN problem statement stub for 3985.
solution/3900-3999/3985.Palindromic Subarray Sum/README_EN.md	Adds EN problem statement stub for 3985.
solution/3900-3999/3984.Divisible Game/README.md	Adds CN problem statement stub for 3984.
solution/3900-3999/3984.Divisible Game/README_EN.md	Adds EN problem statement stub for 3984.
solution/3900-3999/3983.Subsequence After One Replacement/README.md	Adds CN problem statement stub for 3983.
solution/3900-3999/3983.Subsequence After One Replacement/README_EN.md	Adds EN problem statement stub for 3983.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/Solution.ts	Adds TypeScript implementation for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/Solution.py	Adds Python implementation for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/Solution.java	Adds Java implementation for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/Solution.go	Adds Go implementation for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/Solution.cpp	Adds C++ implementation for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/README.md	Adds CN writeup + multi-language code snippets for 3982.
solution/3900-3999/3982.Sum of Integers with Maximum Digit Range/README_EN.md	Adds EN writeup + multi-language code snippets for 3982.
solution/3900-3999/3979.Maximum Valid Pair Sum/Solution.ts	Adds TypeScript implementation for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/Solution.py	Adds Python implementation for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/Solution.java	Adds Java implementation for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/Solution.go	Adds Go implementation for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/Solution.cpp	Adds C++ implementation for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/README.md	Expands CN solution writeup + adds code snippets for 3979.
solution/3900-3999/3979.Maximum Valid Pair Sum/README_EN.md	Expands EN solution writeup + adds code snippets for 3979.
solution/3900-3999/3978.Unique Middle Element/Solution.ts	Adds TypeScript implementation for 3978.
solution/3900-3999/3978.Unique Middle Element/Solution.py	Adds Python implementation for 3978.
solution/3900-3999/3978.Unique Middle Element/Solution.java	Adds Java implementation for 3978.
solution/3900-3999/3978.Unique Middle Element/Solution.go	Adds Go implementation for 3978.
solution/3900-3999/3978.Unique Middle Element/Solution.cpp	Adds C++ implementation for 3978.
solution/3900-3999/3978.Unique Middle Element/README.md	Expands CN solution writeup + adds code snippets for 3978.
solution/3900-3999/3978.Unique Middle Element/README_EN.md	Expands EN solution writeup + adds code snippets for 3978.

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