fix(metrics): use vm_stat for accurate macOS memory reporting

[Copilot]

macOS memory appeared halved (~7GB on 14GB runners) because systeminformation calculates available = total - Pages_active, misclassifying wired and compressed memory as available.

macOS memory parser (src/lib.ts)

• Added parseMacOsVmStat() — parses vm_stat output directly, computing available as (free + inactive + purgeable + speculative) × pageSize, matching Activity Monitor's definition
• Handles both 4KB (Intel) and 16KB (Apple Silicon) page sizes dynamically from the vm_stat header
• getMacOsMemory() wrapper with execSync fallback

Metrics collection (src/main/metrics.ts)

• On darwin, calls getMacOsMemory() instead of systeminformation.mem(). Other platforms unchanged.

Test fix (src/main/metrics.test.ts)

• Mocks node:child_process (execSync → controlled vm_stat output) and node:os (totalmem → fixed value) so getMacOsMemory() returns deterministic values on macOS CI runners
• Spreads real node:os exports to preserve EOL etc. for @actions/core

// systeminformation (wrong on macOS):
result.active = Pages_active * pageSize;          // misses wired, compressed
result.available = result.free + result.buffcache; // includes wired as "available"

// New (correct):
const available = (pagesFree + pagesInactive + pagesPurgeable + pagesSpeculative) * pageSize;
return { active: totalMemory - available, available };

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