Add Yearwheel grammar and tests for Langium

[Shahir-47]

📑 Summary

Defines grammar and parser support for the new yearwheel diagram requested in #7421.

This PR adds:

• a new yearwheel language in the parser (module, index, token builder)
• parser registration so parse('yearwheel', ...) works
• syntax support for:
  • diagram start: yearwheel
  • optional base date: baseDate now or baseDate YYYY-MM-DD
  • events: ID("label", "cron")
  • shared metadata: title, accTitle, accDescr
• tests that confirm valid inputs parse and invalid inputs fail

This PR is only for grammar/parsing (no rendering changes yet).

Part of #7421

📏 Design Decisions

• Used the same file structure and parser setup as other diagram types to keep maintenance simple.
• Kept token handling minimal and consistent with existing diagram token builders.
• Focused tests on real user syntax:
  • valid declaration, baseDate, events, and metadata
  • invalid declaration forms
  • invalid baseDate values and ordering
  • malformed event and accessibility lines
• Limited scope to parser work so syntax can be reviewed and merged independently from rendering.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🦋 If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

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Latest commit: c5092cd

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commit: c5092cd

[codecov]

Codecov Report

❌ Patch coverage is 2.32558% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 3.52%. Comparing base (277c496) to head (c5092cd).

Files with missing lines	Patch %	Lines
packages/parser/src/language/yearwheel/module.ts	4.54%	21 Missing ⚠️
packages/parser/tests/test-util.ts	0.00%	9 Missing ⚠️
...ages/parser/src/language/yearwheel/tokenBuilder.ts	0.00%	5 Missing ⚠️
packages/parser/src/parse.ts	0.00%	5 Missing ⚠️
packages/parser/src/language/index.ts	0.00%	1 Missing ⚠️
packages/parser/src/language/yearwheel/index.ts	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #7459      +/-   ##
==========================================
- Coverage     3.52%   3.52%   -0.01%     
==========================================
  Files          509     511       +2     
  Lines        49790   49822      +32     
  Branches       785     788       +3     
==========================================
+ Hits          1754    1755       +1     
- Misses       48036   48067      +31     

Flag	Coverage Δ
unit	3.52% <2.32%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
packages/parser/src/language/index.ts	0.00% <0.00%> (ø)
packages/parser/src/language/yearwheel/index.ts	0.00% <0.00%> (ø)
...ages/parser/src/language/yearwheel/tokenBuilder.ts	0.00% <0.00%> (ø)
packages/parser/src/parse.ts	1.12% <0.00%> (-0.07%)	⬇️
packages/parser/tests/test-util.ts	1.20% <0.00%> (-0.15%)	⬇️
packages/parser/src/language/yearwheel/module.ts	4.54% <4.54%> (ø)

... and 1 file with indirect coverage changes

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No changes detected	-	Mar 15, 2026, 9:07 AM

[mrrudin]

Happy to see that you picked this up!

Please tell me when there is something to test, and I will try to help out!

[Shahir-47]

Happy to see that you picked this up!

Please tell me when there is something to test, and I will try to help out!

Currently, I have just defined the grammar and parsing for this new diagram. I want the maintainers to confirm if the grammar and parsing is acceptable before I move on to the rendering step.