Extract sbuf library and integrate unit testing

[Zaid-maker]

Summary by CodeRabbit

• New Features
  • Automated unit tests now execute in CI/CD pipelines before kernel builds
  • Host-side testing infrastructure provided for developers
• Documentation
  • Added unit test execution instructions and updated regression testing checklist

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a7a8f8fe-63f9-4938-ab6c-6a8c759d7229

📥 Commits

Reviewing files that changed from the base of the PR and between 9af0081 and 8821d61.

📒 Files selected for processing (8)

• .github/skills/kernel-from-start/SKILL.md
• README.md
• kernel/Makefile
• kernel/VERSION
• kernel/src/sbuf.c
• kernel/tests/sbuf_integration_test.c
• kernel/tests/sbuf_test.c
• site/index.html

📝 Walkthrough

Walkthrough

The PR extracts string buffer formatting utilities from kernel.c into a reusable sbuf library (header, implementation, tests), refactors kernel.c to use the new functions, and integrates host-side unit testing into the build system and CI workflows.

Changes

Cohort / File(s)	Summary
String Buffer Library kernel/src/sbuf.h, kernel/src/sbuf.c	New string buffer API with functions for resetting buffers, appending characters/strings, and formatting unsigned integers in decimal and hexadecimal forms with capacity/bounds checking.
Test Suite kernel/tests/sbuf_test.c	New host-side unit tests validating sbuf functionality including character/string/integer appends, hexadecimal formatting, and capacity overflow guards with pass/fail assertions.
Kernel Refactoring kernel/src/kernel.c	Removed 6 local string-building helper functions and replaced with include of sbuf.h to use externally provided implementations.
Build System kernel/Makefile	Added host compiler support (HOST_CC, HOST_CFLAGS), included src/sbuf.c in kernel sources, and introduced test phony target to build and run sbuf_test.
CI Workflows .github/workflows/build-kernel.yml, .github/workflows/release-kernel.yml	Added conditional and unconditional make test steps (with CROSS_COMPILE=) executing before kernel binary/ISO builds.
Documentation README.md	Added "Run Unit Tests" section documenting make -C kernel test invocation and updated regression checklist to confirm sbuf.c unit test coverage.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

A rabbit hops through kernel code with glee,
Extracted buffers, now reusable and free!
Tests verify each character appends right,
Hex formatting shines with bounds-check delight,
Hop-hop, the CI pipeline tests with care! 🐰✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title Check	✅ Passed	Title check skipped as CodeRabbit has written the PR title.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch migrate-heap-backed-allocation

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@kernel/src/sbuf.c`:
- Around line 17-30: sbuf_append_str currently returns early when *len + 1u >=
cap without null-terminating the buffer; update the function so that before any
early return (inside the if checking capacity) you write buffer[*len] = '\0'
(using the current *len value) to ensure the buffer is always NUL-terminated on
partial writes, leaving the rest of logic in sbuf_append_str unchanged.

🪄 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: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1bbb4641-f282-4134-a091-f601804e5d4f

📥 Commits

Reviewing files that changed from the base of the PR and between 0bec007 and 9af0081.

📒 Files selected for processing (8)

• .github/workflows/build-kernel.yml
• .github/workflows/release-kernel.yml
• README.md
• kernel/Makefile
• kernel/src/kernel.c
• kernel/src/sbuf.c
• kernel/src/sbuf.h
• kernel/tests/sbuf_test.c