Restructure into topic folders and add 100+ questions with examples

Corrections (about 60 answers), including:
- "Can you extend an interface?" said No; interfaces can extend interfaces
- protected access described as visible to non-subclasses outside the package
- ClassCastException called checked and ClassNotFoundException unchecked
- "Running" listed as a Java thread state; Thread.State has six states
- Implicit/explicit casting mislabelled as down/up casting; wrong widening order
- Deserialization constructor and static-field answers were wrong
- try-with-resources: resources close before catch runs, not after
- Exception handling pattern attributed to a non-existent GoF pattern
- OutOfMemory answer copied from Confluence docs and used PermGen flags
- "Will this code compile?" answered No for code that compiles
- equals() example threw on null/other types; hashCode called unique
- Outdated APIs: deprecated wrapper constructors, finalize(), javax

Structure:
- Every question tagged [Easy]/[Medium]/[Hard], ordered easy to hard
- Blank line after every </summary> so markdown renders on GitHub
- README rebuilt with level counts and a study path

New: core Java essentials, advanced exceptions, collection complexity,
threading pitfalls and executors, Java 8 advanced, generics erasure and
wildcards, and DSA problems 21-33 (heaps, graphs, DP, sliding window).

All new DSA solutions and stated example outputs compiled and checked
on JDK 21.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Jgqi4T7d6q9GRmzBXKCTm

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Jgqi4T7d6q9GRmzBXKCTm

Fix incorrect answers, add difficulty levels and new topics

Files were sorted alphabetically, so advanced topics (e.g. advanced-oops,
concurrency-advanced) appeared before the basics. Each file now has a
numeric prefix in study order, DSA starts with classic programs and ends
with LLD, and cross-references and the README index are updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Jgqi4T7d6q9GRmzBXKCTm

Order files in every folder from basic to advanced