fix: correct contributor growth calculations

[nehayadav827]

Description

This PR fixes a data correctness issue in src/lib/data-processing.ts where contributorGrowth always returned a single hardcoded entry instead of a real per-week contributor growth breakdown.

This directly relates to the Builder Intelligence analytics layer described in Issue #68.

---

Problem

Previously, processData() returned:

contributorGrowth: [{
  week: new Date().toISOString().split('T')[0],
  newContributors: activeContributors,
  returningContributors: 0,
  totalActive: activeContributors
}]

This caused multiple analytics issues:

• no weekly contributor growth history
• returningContributors always remained 0
• newContributors incorrectly used the all-time contributor count
• charts received a single synthetic entry instead of week-by-week trends

---

Fix

Added calculateContributorGrowth() to:

• sort program weeks chronologically
• track contributor first appearances
• distinguish new vs returning contributors
• calculate per-week active contributor totals
• return one contributor growth entry per program week

processData() now uses:

contributorGrowth: calculateContributorGrowth(csvData)

---

Changes

• Added calculateContributorGrowth()
• Replaced hardcoded contributor growth object
• Added chronological weekly contributor tracking
• Fixed returning contributor calculations
• Kept implementation scoped to src/lib/data-processing.ts

---

Validation

A standalone local validation script was used during development to verify:

• broken behaviour reproduction
• correct weekly contributor aggregation
• new vs returning contributor tracking
• chronological ordering
• edge-case handling

Validation result:

12 passed   0 failed

Validation Output

Also ran:

git diff --check

No formatting or whitespace issues were reported.

---

Additional Notes

Current repository checks still contain unrelated existing blockers:

• npx tsc --noEmit --pretty false fails on existing test typing/fixture issues in src/lib/__tests__/utils.test.ts
• npm test -- --runInBand src/lib/__tests__/utils.test.ts fails on an existing expectation mismatch unrelated to this change

No breaking changes introduced.

---

Checklist

• I have tested this code
• I have linked relevant issue(s)
• I followed the code style of the project
• I reviewed my own code

---

Relates to #68

[vercel]

@nehayadav827 is attempting to deploy a commit to the SEETA's projects Team on Vercel.

A member of the Team first needs to authorize it.