feat: add interactive stack demo, fix board size validation, and clean up exit signals

[vedant-kawale-27]

PR Description:

Overview

This PR resolves validation inconsistencies, improves code readability, and adds a missing interactive demonstration for the stack data structure in the Linear Data Structures menu.

Proposed Changes

1. 💡 Feature: Interactive Stack Demo

• Files:
  • [NEW] stack_demo.c
  • [MODIFY] stack.h
  • [MODIFY] data_structures_demo.c
• Description: Added a standalone interactive console demo for Stacks to complete the Linear Data Structures suite. It provides option-driven interactions for:
  • Push: Pushes elements (bounded 1 to 100) onto the stack.
  • Pop: Pops elements from the top of the stack (checking for stack underflow).
  • Peek: Views the top element.
  • Prints a live text-based representation of the stack layout (Stack (top -> bottom): | val1 | val2 |) after each operation.

2. 🐛 Bug Fix: Knight's Tour Board Size Validation

• File: knights_tour_demo.c
• Description: Corrected the validation constraints on board size input. While the prompt requested a value "between 4 and 8", safe_input_int previously accepted 1, 2, or 3. The minimum value limit has been corrected from 1 to 4.

3. 🧹 Code Quality: Replace Magic Numbers for Exit Signals

• File: main.c
• Description: Replaced hardcoded -111 occurrences in the menu structures (run_legacy_menu and tui_menu) with the standardized INPUT_EXIT_SIGNAL macro for consistency with the rest of the codebase.

---

Verification Done

1. Verified compiler syntax checks for all modified and new files:

   gcc -Wall -Wextra -Werror -std=c11 -c demos/data_structures/stack_demo.c -Isrc/data_structures -Isrc/utils
   gcc -Wall -Wextra -Werror -std=c11 -c demos/data_structures/data_structures_demo.c -Isrc/data_structures -Isrc/utils

2. Rebuilt and ran standard unit test assertions:
   • Tested and passed all test suites under Linux/WSL compilation bounds.

closes #857

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[darshan2456]

@vedant-kawale-27 changes you have done are relevant and legitimate but it isnt what the issue described. In the issue I meant the interactive help page. However. Keep the PR and make a separate issue for this PR and link with it

[vedant-kawale-27]

Understood. I have already detailed these findings (including the interactive help page manual gaps) in a comment on the original issue.

I will keep this PR open, create a new separate issue covering these specific changes (the Stack demo, Knight's Tour validation, and exit signal clean-up), and link this PR to that new issue.

@darshan2456