fix(aggregate): include first row in SUM and coerce numeric strings

[namannitr]

Summary

• src/aggregate.ts already seeded SUM from the first in-group value (PR Fix for #1 #3 / issue initial value of sum should be the first value instead of 0 #1), but dist/ecSimpleTransform.js was never rebuilt, so npm and jsDelivr still ship SUM: function () { return 0; }. Singleton groups therefore SUM to 0, and multi-row groups skip the first row.
• Arithmetic methods now coerce cells with Number(...) so object-row sources like { billAmount: '400' } add as numbers instead of concatenating strings (issue Sum method not giving correct result #6).
• Rebuilt dist/ (including the min bundle jsDelivr points at) and added npm test coverage for both cases.

Test plan

• npm run release && npm test — covers singleton SUM, multi-row SUM including the first row, and the billAmount string fixture from Sum method not giving correct result #6 (Mar=1000, April=210, August=148)
• After merge, please publish a new npm version so CDN users pick this up (jsDelivr serves dist/ecSimpleTransform.min.js)

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