feat: add threshold (black & white) filter with -t flag

[Satvik-Singh192]

Description

Implemented a new Threshold Filter (-t flag) that converts each pixel of an image to pure black or white based on average intensity.

The filter was added to helpers.c, declared in helpers.h, registered in filter.c, and documented in README.md.

Usage:

./filter -t input.bmp output.bmp

This applies the black-and-white threshold effect.

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Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

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Issues

Closes #18 — Feature: Threshold Filter (Black & White)

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Checklist

• I have read the Contributing Guidelines.
• Code compiles successfully without modifying existing logic.
• Feature tested locally using sample .bmp files.
• README updated with new filter usage.