fix: Correct getDayOfWeek() conversion for Sunday (fixes #42)

[richardwooding]

Summary

Fixes #42 - getDayOfWeek() now correctly returns 6 for Sunday instead of -1.

Problem

• Root cause: PostgreSQL EXTRACT(DOW) returns 0 for Sunday, but simple subtraction (- 1) resulted in -1
• Expected: CEL uses ISO 8601 weekday numbering (0=Monday, 6=Sunday)
• Impact: Medium severity - incorrect day-of-week values for Sunday

Solution

Applied modulo arithmetic to correctly map PostgreSQL DOW to CEL ISO 8601:

(EXTRACT(DOW FROM ...) + 6) % 7

Conversion table:

• Sunday (DOW=0): (0 + 6) % 7 = 6 ✅
• Monday (DOW=1): (1 + 6) % 7 = 0 ✅
• Tuesday (DOW=2): (2 + 6) % 7 = 1 ✅
• Wednesday (DOW=3): (3 + 6) % 7 = 2 ✅
• Thursday (DOW=4): (4 + 6) % 7 = 3 ✅
• Friday (DOW=5): (5 + 6) % 7 = 4 ✅
• Saturday (DOW=6): (6 + 6) % 7 = 5 ✅

Changes

1. timestamps.go:

   • Wrap EXTRACT(DOW ...) in parentheses for getDayOfWeek
   • Apply (+ 6) % 7 formula instead of simple - 1
   • Use switch statement for better code organization

2. pg/postgres17_compat_test.go:

   • Added test for Monday (CEL value 0)
   • Added test for Sunday (CEL value 6) - verifies the fix
   • Added test for Thursday (CEL value 3)
   • All tests run against real PostgreSQL 17 database

3. docs/operators-reference.md:

   • Updated conversion formula documentation

Test Plan

• All existing tests pass
• New tests verify Monday, Sunday, and Thursday conversions
• Integration tests run against PostgreSQL 17
• Lint checks pass
• Code formatted with make fmt

Verification

make fmt
make lint
make test

All checks pass ✅

🤖 Generated with Claude Code