fix: replace git submodule with Julia Artifacts for test fixtures

[s-celles]

Git submodules are incompatible with Julia General registry (git archive doesn't include submodule contents, causing tree-sha1 mismatch).

• Add test/Artifacts.toml with lazy toon_spec artifact (v3.0.1)
• Update test_spec_fixtures.jl to use artifact instead of submodule
• Add LazyArtifacts to test dependencies
• Remove git submodule and deprecated download_fixtures.jl

Linked Issue

Helps #4 and #3

Closes #

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: 3.0.1

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

[s-celles]

This one pass but should fail

     Testing Running tests...
Test Summary:             | Pass  Total   Time
Aqua.jl Quality Assurance |   22     22  17.0s
 Downloading artifact: toon_spec
     Failure artifact: toon_spec
 Downloading artifact: toon_spec
┌ Warning: TOON spec fixtures not available at: /home/runner/.julia/artifacts/fd47fe7b2c0ce1b72c4432d053f76354c1fda790/tests/fixtures
└ @ Main ~/work/ToonFormat.jl/ToonFormat.jl/test/test_spec_fixtures.jl:28
Test Summary:             | Pass  Broken  Total   Time
ToonFormat.jl - All Tests | 1749       1   1750  38.0s
Test Summary:               | Pass  Total  Time
ToonFormat.jl - Basic Tests |   40     40  0.9s
     Testing ToonFormat tests passed

[s-celles]

LGTM now...

https://github.com/toon-format/ToonFormat.jl/actions/runs/20456298199/job/58779037140?pr=7

     Testing Running tests...
Test Summary:             | Pass  Total   Time
Aqua.jl Quality Assurance |   22     22  17.0s

======================================================================
Official Fixtures: 349/349 passing, 0 failing, 0 skipped (100.0%)
======================================================================

Test Summary:             | Pass  Total   Time
ToonFormat.jl - All Tests | 2098   2098  45.5s
Test Summary:               | Pass  Total  Time
ToonFormat.jl - Basic Tests |   40     40  0.2s
     Testing ToonFormat tests passed