Add comprehensive exercise explanations for all 21 Programming Fundamentals Exam exercises

[Copilot]

Repository lacked detailed explanations for the exam exercises beyond code solutions. Added comprehensive documentation covering all 21 exercises across 4 difficulty levels.

Changes

• Created EXERCISES_EXPLAINED.md (1,053 lines)

  • Problem statements and requirements for each exercise
  • Step-by-step logic breakdowns with input/output examples
  • Key concepts, edge cases, and implementation notes
  • Organized by difficulty level with summary comparison table

• Updated repository READMEs

  • Corrected exercise counts (18 → 21) in main and exam READMEs
  • Added cross-references to comprehensive guide
  • Updated Level 2 (5 → 6 exercises) and Level 4 (3 → 5 exercises) listings

Example

Each exercise explanation follows a consistent structure:

## 12. inter
**Difficulty**: ⭐⭐
**File**: `inter.c`

### Problem
Display characters appearing in both strings, no duplicates, first string order.

### Logic Breakdown
Input: "./inter" "padinton" "paqef..."
- Track seen characters in array[256]
- For each char in first string:
  - If exists in second AND not printed → print, mark seen
Output: "padinton"

### Key Concepts
- Set intersection via nested loops
- Duplicate prevention with seen[] array
- Order preservation from first string

Documentation serves as both learning resource and quick reference for exam preparation.

Original prompt

explain to me every ex

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