feat(std): add bounds-tracked string builder

[NKS01X]

Resolves #9

The Problem

Standard C string manipulation functions (strcat, sprintf, etc.) are notoriously prone to buffer overflows and silent truncation. lib0xc needed a safe, bounds-tracked mechanism for dynamically constructing strings within statically-allocated buffers.

The Solution

This PR introduces xc_str_builder_t and its associated API to the std module. It provides a lightweight, bounds-checked wrapper around a character array that safely handles appends and formatting without risking memory corruption.

Key Features Implemented:

• xc_str_builder_init: Safely wraps a statically-allocated buffer, ensuring it is properly null-terminated from the start.
• xc_str_builder_append: Appends strings while strictly enforcing capacity limits.
• xc_str_builder_appendf: Safely formats data using vsnprintf. Includes specific logic to detect silent truncation and actively "undo" partial writes to prevent corrupting the buffer state.
• xc_str_builder_append_char: Fast, O(1) safe single-character appends.
• Buffer Management: Includes clear and truncate utilities for safe buffer reuse.

Testing

• Created a comprehensive test suite in 0xtest/unit/test_str_builder.c.
• Integrated directly with the internal UNIT_TEST framework (0xc/sys/check.h).
• Verified 100% pass rate locally on Clang.
• Tests specifically cover bounds overflow prevention and partial-write reversion during formatted truncation.

Notes for Reviewers

All formatting strictly adheres to the project's C guidelines, including Allman-style bracing, padded function parameters, and the required Microsoft copyright headers.

[NKS01X]

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