Remove length marker option

[mattt]

Linked Issue

Closes #13

Description

This PR removes the length marker option, which was removed from the v3 spec.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Changes Made

• Remove the lengthMarker property and nested LengthMarker type in TOONEncoder
• Remove discussion of length markers from README
• Remove invalidated tests.

SPEC Compliance

• This PR implements/fixes spec compliance
• Spec section(s) affected:
• Spec version: v3

Testing

• All existing tests pass
• Added new tests for changes
• Tests cover edge cases and spec compliance

Pre-submission Checklist

• My code follows the project's coding standards
• I have run code formatting/linting tools
• I have added tests that prove my fix/feature works
• New and existing tests pass locally
• I have updated documentation if needed
• I have reviewed the TOON specification for relevant sections

Breaking Changes

• No breaking changes
• Breaking changes (describe migration path below)

We will release a new minor / pre-1.0 version.

Additional Context

[Copilot]

Pull request overview

This PR removes the lengthMarker encoder option to align with the TOON v3 specification, which no longer includes this feature. This is a breaking change that simplifies the encoder API by removing an optional feature that prefixed array lengths with a # character in the output format.

Key changes:

• Removed the lengthMarker property and LengthMarker enum from TOONEncoder
• Updated all formatHeader function calls to remove the lengthMarker parameter
• Removed encoder tests specific to the length marker feature
• Removed documentation section explaining length marker usage

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
Sources/ToonFormat/Encoder.swift	Removed LengthMarker enum, lengthMarker property, and updated all formatHeader calls to exclude the length marker parameter
Tests/ToonFormatTests/EncoderTests.swift	Removed 6 test functions that tested the length marker feature with various configurations
Tests/ToonFormatTests/DecoderTests.swift	Removed round-trip test for length marker encoding/decoding
README.md	Removed documentation section explaining the length marker feature with code examples

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