Improve package output for tree shaking

[krowvin]

Summary

Improves the library package output so consumers can import smaller parts of Groundwork without pulling the whole bundle:

• publish preserved ESM modules alongside the existing UMD build
• expose component, composite, hook, and utility subpaths
• move CSS to an explicit stylesheet entry
• lazy-load heavier footer image assets

Bundle check

Package pack size:

Package	Before	After
Packed tarball	4.65 MB	4.55 MB
Unpacked package	7.45 MB	7.05 MB

Representative downstream bundle:

Import	Before	After	Gzip before	Gzip after
Button from @usace/groundwork	368.9 KB	244.0 KB	115.1 KB	75.7 KB

Validation

• npm run build-lib
• npm run build
• packed the library and tested representative downstream bundles

Related: #191, #218, #221, #179

[github-actions]

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