Expose time globals to Lua raters and generators

[coccyx]

Summary

• Expose now, beginTime, and endTime as Lua globals in script raters, enabling time-aware rate decisions (operating hours, weekly ramp-up curves)
• Expose beginTime and endTime to Lua generators for timeline-aware event scheduling
• 3 new tests covering all new globals; all existing tests pass with no regressions

Test plan

• go test ./rater/ ./generator/ — all tests pass
• Verify now is accessible in Lua rater scripts and reflects the current interval time
• Verify beginTime/endTime are accessible in both generators and raters when begin/end are configured
• Verify beginTime/endTime are not set when begin/end are omitted (no nil errors)

🤖 Generated with Claude Code

[coveralls]

coverage: 71.853%. first build
when pulling 2900825 on feature/lua-time-expose
into 1f35225 on dev.