WEBDEV-8506: Add conventions for non-component modules

[jbuckner]

Summary

Establishes the directory layout, package.json exports patterns, and test conventions for non-component modules (services, parsers, models, type utilities) ahead of consolidating metadata-service and its dependency tree into elements.

No code is moved in this PR — it's documentation + the routing rules that future migration PRs will fill in.

Changes

• README.md — new "Adding a Service or Library Module" section parallel to "Adding a Component", documenting the new top-level directories (src/services, src/parsers, src/models, src/types), their purposes, structure, exports, and test patterns.
• package.json — adds subpath exports for the four new module directories. Patterns mirror the existing ./labs/* pattern.

Why a separate ticket?

The next four tickets (WEBDEV-8507, WEBDEV-8508, WEBDEV-8509, WEBDEV-8510) each move a single repo into elements/src/. Landing the conventions first means each migration PR is straightforward "move code into the agreed location" without re-litigating layout.

Test plan

• npm run build passes
• Reviewers: confirm the proposed directory split (services / parsers / models / types) reads naturally — easy to redo before any code migrates if the names should change.

QA waived — docs + config only, no runtime artifact. qa-waived label applied at Code Review transition.

🤖 Generated with Claude Code

[github-actions]

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.75%. Comparing base (9a3b0f3) to head (a60acb4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
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  Coverage   78.75%   78.75%           
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  Files          16       16           
  Lines         645      645           
  Branches      168      168           
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  Hits          508      508           
  Misses         95       95           
  Partials       42       42           

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[rebecca-shoptaw]

Thanks for doing this!! In the interest of keeping things simple and avoiding a ballooning list of sections, I'd personally prefer a simple division like this: services / utils (standalone functions, incl parsers) / types (types, models, etc.)