research(build): migrate to tsdown bundler

[overbalance]

Which problem is this PR solving?

Research for #4898

ESM builds don't work in Node.js or Deno due to:

1. The module export condition is non-standard (bundler-only)
2. ESM output lacks .js extensions required by Node.js

Short description of the changes

• Migrate build from tsc to tsdown - Replaces multi-target tsc builds (CJS/ESM/ESNext) with tsdown bundler
• Dual CJS/ESM output - Outputs .cjs and .mjs files with proper extensions
• Fix exports field - Uses standard import/default conditions instead of module
• Keep esnext condition - Points to .mjs for consumers wanting ES2022+ syntax
• Preserve browser field - Maintains backwards compatibility for bundlers
• Nx inferred tasks - Adds scripts/nx/tsdown-plugin.js for compile targets
• ESLint CJS configs - Renames .eslintrc.js to .eslintrc.cjs for ESM compatibility

Export condition order: types → esnext → import → default

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• npm run compile passes (44 packages)
• npm run lint passes
• ESM and CJS imports verified manually

Checklist:

• Followed the style guidelines of this project
• Changelog entries added
• Documentation has been updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.41%. Comparing base (0a1a631) to head (e77327d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6201   +/-   ##
=======================================
  Coverage   95.41%   95.41%           
=======================================
  Files         317      317           
  Lines        9460     9460           
  Branches     2199     2199           
=======================================
  Hits         9026     9026           
  Misses        434      434           

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[overbalance]

Interesting research but tsdown doesn't preserve barrel files so we would need to modify how node/browser entry files are constructed.