Add LSB parsing support to bitmask

[FlickerSoul]

Goal

This PR adds LSB parsing support to bitmask.

The bitmask parsing is defaulted to MSB (BE). To opt in LSB (LE) parsing, one needs to specify in the @ParseStrucut, @ParseEnum, and @ParseBitmask macro with bitEndian: .little.

Design

This PR replaces the old RawBitsInteger-based approach with a new RawBitsSpan type and uses it to enable endianness-aware bit-level parsing.

RawBitsSpan

RawBitsSpan is a ~Escapable, ~Copyable span type that provides a zero-copy view into a contiguous byte buffer with bit-level granularity. It tracks a _bitOffset and _bitCount into an underlying Span.

Key operations:

• load(as:bitCount:) — extracts bits as a right-aligned fixed-width integer
• extracting(first:) / extracting(last:) — non-mutating sub-span extraction
• slicing(first:) / slicing(last:) — mutating extraction that consumes bits from the span

This replaces the previous design where all bits were pre-extracted into a FixedWidthInteger (RawBitsInteger associated type). The span-based approach removes the 64/128-bit size limit, avoids copying bits into an integer container, and decouples conforming types from a specific integer width.

Endianness flow

The bitEndian parameter flows through the system as follows:

1. Declaration — user specifies bitEndian: .little on the macro attribute
2. Macro expansion — MacroConfigurationVisitor extracts the value and stores it in AccessorInfo
3. Code generation — the macro generates RawBitsSpan initialization and field slicing calls:
   - Big endian (default): span starts at bit offset 0, fields are sliced with __slicing(unchecked: (), first:), consuming from MSB toward LSB
   - Little endian: span starts at bit offset totalBytes * 8 - totalBits, fields are sliced with __slicing(unchecked: (), last:), consuming from LSB toward MSB
4. Runtime — __createFromBits adjusts the span when fieldRequestedBitCount > typeBitCount, extracting from first (big) or last (little) to select the significant bits

ExpressibleByRawBits protocol change

The protocol no longer has an associated RawBitsInteger type. The initializer now takes borrowing RawBitsSpan:

public protocol ExpressibleByRawBits {
    init(bits: borrowing RawBitsSpan) throws
}

Conformances for Bool, UInt8, Int8, and other integer types extract only the bits they need via RawBitsSpan.load().

Other changes

• CI updated to use Swiftly
• Benchmarks disabled by default
• Removed __maskParsing and ExtractBitsAsIntegerTests / MaskParsingTests (superseded by RawBitsSpan and new tests)
• Refactored macro utility naming (MacroConfigurationVisitor, extractMacroConfiguration)
• Added comprehensive test coverage for both endianness modes across bitmask, enum, and struct parsing

[Copilot]

Pull request overview

This PR adds Little Endian (LSB-first) parsing support to bitmask operations in BinaryParseKit. The default behavior remains Big Endian (MSB-first), but users can now opt into Little Endian parsing by specifying bitEndian: .little in the @ParseStruct, @ParseEnum, and @ParseBitmask macros.

Key changes:

• Introduces RawBitsSpan struct for efficient bit-level operations without copying
• Updates ExpressibleByRawBits protocol to use RawBitsSpan instead of raw integer types
• Adds bitEndian parameter to parsing macros with validation for .big or .little values
• Comprehensive test coverage for both big and little endian scenarios

Reviewed changes

Copilot reviewed 35 out of 35 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Sources/BinaryParseKit/Utils/RawBitsSpan.swift	New core type for representing bit spans with efficient slicing operations
Sources/BinaryParseKit/Protocols/BitmaskParsable.swift	Updated protocol to use RawBitsSpan instead of RawBitsInteger
Sources/BinaryParseKit/Utils/ParsingUtils.swift	Refactored utility functions to work with RawBitsSpan and support bitEndian parameter
Sources/BinaryParseKitMacros/Macros/Supports/MacroAccessorVisitor.swift	Added bitEndian configuration extraction and validation
Sources/BinaryParseKitMacros/Macros/Supports/Utilities.swift	Updated mask group generation to support big/little endian slicing
Tests/BinaryParseKitTests/Utils.swift	New utility for creating ParserSpan from integers
Tests/BinaryParseKitTests/Parsing/*.swift	Comprehensive test updates with little endian test cases
Benchmarks/*	Updated benchmarks to use new RawBitsSpan API

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.