Encode dictionary keys in deterministic order

[mattt]

Linked Issue

Alternative to #22

Description

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

SPEC Compliance

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

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)

Additional Context

[Copilot]

Pull request overview

This PR aims to ensure deterministic output when encoding Swift Dictionary values with TOONEncoder, aligning behavior with the linked spec-compliance issue/PR (#22).

Changes:

• Sort encoded dictionary keys lexicographically to make output deterministic.
• Add tests to validate deterministic dictionary ordering and to validate the DictionaryCodingKey-based detection heuristic.
• Document object key ordering behavior in the README.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
Tests/ToonFormatTests/EncoderTests.swift	Adds regression tests for deterministic dictionary ordering and the dictionary-key heuristic.
Sources/ToonFormat/Encoder.swift	Implements dictionary detection heuristic and uses sorted keys for dictionary encoding.
Sources/ToonFormat/Value.swift	Removes unused Value.from(_:) factory logic.
README.md	Documents preserved struct field ordering and lexicographic sorting for dictionary keys.

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