feat: track spawned processes (execve count) per variation

[vltbaudbot]

Adds a new Spawned Processes data point that counts the number of processes each package manager spawns (via execve syscalls) during install. This varies by state/variation, so it's captured for each one — just like package count data.

How it works

After the hyperfine benchmark completes for each variation, a separate strace run is performed for each PM under the same state:

1. scripts/process-count.sh — wraps a single install with strace -f -e trace=execve, counts execve() calls in the output
2. scripts/collect-process-count.js — aggregates per-PM counts into process-count.json (same format as package-count.json: { "npm": { "count": 42 }, ... })
3. collect_process_count() in common.sh — orchestrates the strace runs, using the variation's BENCH_PREPARE_BASE + per-PM setup commands to replicate the correct state

The strace run is separate from the timed benchmark so it doesn't affect timing measurements.

Backend

• New scripts: process-count.sh, collect-process-count.js
• common.sh: extracted bare BENCH_INSTALL_* commands (no log redirection) for reuse by both hyperfine and strace; added collect_process_count()
• All 8 standard variation scripts (clean, cache, lockfile, node_modules, combos): call collect_process_count after collect_package_count
• process-results.sh: copies process-count.json to dated + latest results
• setup.sh: ensures strace is installed

Frontend

• New types: ProcessCountData, ProcessCountTableRow
• New hook: use-process-count-data.ts (fetches /latest/<fixture>-<variation>-process-count.json)
• New component: ProcessCountTable — sortable table with Terminal icon, locale-formatted numbers
• Integrated into variation page below the Package Count Data section
• New utility: getAvailablePackageManagersFromProcessCount

Notes

• Currently scoped to the 8 standard install variations (same scope as package count). Registry and run variations are not included yet.
• If strace is unavailable, the collection step is silently skipped (no-op).
• The strace log files are cleaned up after counting to save disk space.