Minor performance improvement in BitmapByteQRCode + clarify padding calculation

[KrisVandermotten]

Summary

This PR fixes/implements the following bugs/features:

• Fixes a bug in BMP padding calculation when pixelsPerModule is odd.
• Minor optimization using BinaryPrimitives.WriteInt32LittleEndian(Span, Int32) on modern .NET

Closing issues

Fixes #697

Summary by CodeRabbit

• Bug Fixes
  • Improved QR bitmap generation across supported platforms for more consistent output.
  • Corrected bitmap row padding calculations to ensure proper byte alignment.
  • Enhanced binary integer writing on newer .NET Core versions to reliably produce the expected byte order.

[coderabbitai]

📝 Walkthrough

Walkthrough

Bitmap QR generation now calculates BMP row padding from the row’s byte length and validates its alignment. Integer serialization uses BinaryPrimitives on supported .NET Core targets while retaining the existing fallback implementation.

Changes

Bitmap encoding

Layer / File(s)	Summary
Row padding alignment QRCoder/BitmapByteQRCode.cs	Computes padded row length with bitwise rounding and adds a debug assertion for valid 4-byte alignment.
Little-endian integer serialization QRCoder/BitmapByteQRCode.cs	Adds the conditional binary primitives import and uses BinaryPrimitives.WriteInt32LittleEndian for supported .NET Core targets, with manual byte splitting retained elsewhere.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested reviewers: gfoidl

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The row padding calculation is corrected in BitmapByteQRCode.GetGraphic as required by issue #697.
Out of Scope Changes check	✅ Passed	No unrelated changes are evident; the added optimization and assertion stay within the BitmapByteQRCode fix.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the padding calculation fix and the small performance optimization in BitmapByteQRCode.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@QRCoder/BitmapByteQRCode.cs`:
- Around line 75-78: Update the padding calculation in the BitmapByteQRCode
row-building logic to use the 24-bit row byte count, 3 * sideLength, rather than
sideLength itself; revise the Debug.Assert invariant to validate that byte count
plus padding is divisible by 4, and add regression coverage with an odd
pixelsPerModule value such as 3.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: bf0cb601-4c81-4763-8114-7ed94d213c46

📥 Commits

Reviewing files that changed from the base of the PR and between 4b97eb1 and 348e0a9.

📒 Files selected for processing (1)

• QRCoder/BitmapByteQRCode.cs

[Shane32]

• Fixes a bug in BMP padding calculation when pixelsPerModule is odd.

Note: An extensive analysis of the old and new calculation code by ChatGPT determined that it is mathematically equivalent for 24bpp images, but the new calculation better represents the intent, and is more correct if the data wasn't 24bpp.