longestCommonSubsequenceRecursive: wrong result when strings contain ':' and exponential time when there is no common character #2233

Two bugs in src/algorithms/sets/longest-common-subsequence/longestCommonSubsequenceRecursive.js.

1. Wrong result. The memo key is `${s1}:${s2}` (lines 19 and 30), which is ambiguous when the strings contain :. For example, (':a:a', '::') and (':a', 'a:::') both map to ":a:a:::".

longestCommonSubsequenceRecursive('::a:a', 'aa:::');  // 'aa'   expected length 3 (':::')

This is the minimal case found by exhaustive search.

2. Exponential time. if (memo[key]) treats a memoised '' as a cache miss, so any pair of strings with no common characters is never memoised.

Fix: key on the suffix lengths (${s1.length},${s2.length}) and check key in memo. With both fixes, n = 13 takes 2.6 ms and n = 200 takes 20 ms.

Found during an independent property-based audit of this repo (full report and reproducible harness: https://github.com/devladpopov/algorithm-autopsy/tree/main/autopsy-02).