fix: add start tag output to capturePerf for throughput calculation

[Saga4]

Summary

• Adds start tag output (!$######...######$!) to capturePerf before timing measurement
• Required for throughput calculation and timing marker matching

Problem

capturePerf was only outputting end tags with timing data (!######...######!), but not start tags. This caused:

1. Throughput calculation to fail (requires counting both start and end tags)
2. Timing marker matching in parse_test_output.py to fail (expects start tags to exist)

Solution

Added start tag output before timing measurement in capturePerf, matching the behavior of the capture function:

// Print start tag (needed for throughput calculation and timing marker matching)
console.log(`!$######${testStdoutTag}######$!`);

Test plan

• Added test file runtime/__tests__/capturePerf-start-tags.test.js
• Test verifies both start and end tags are output
• Test verifies tag format is correct
• Manual testing with Vitest-based projects

Generated with Claude Code

[Saga4]

Closing - needs more verification before merging