Show active sandbox count and observed memory

[sam2tom]

Summary

• replace the Runtime Dashboard's Compute uptime trend with an Active Sandboxes time series
• deduplicate active counts and observed memory by runtime_allocation_id
• keep unavailable covered samples visible as zero while preserving true no-coverage gaps
• preserve PostgreSQL durable Runtime history and the latest Dashboard/Session caching behavior from main

Semantics

• one active managed allocation contributes one sandbox, regardless of duplicate Session observations
• memory totals include only allocations with an observed usage/limit pair
• allocation identity does not depend on started_at; uptime remains a current/detail value only
• provider-neutral projection continues to support Docker and microsandbox observations

Validation

• git diff --check
• focused Dashboard Vitest: 48 passed
• focused final zero-transition Vitest: 22 passed
• Web Vitest: 70 files / 690 tests passed
• TypeScript check passed
• Vite production build passed
• Playwright acceptance: 117 passed
• Go, DB, Agents API, Runtime History, Runtime observation, Docker, microsandbox, Sandbox Node, and SDK stages of make check passed
• independent blind review completed; one missing zero-transition test was fixed and re-review found no remaining blocker

Known platform gate

The final Rust executor stage of make check is blocked on macOS because current main compiles Linux-only rustix APIs (inotify, mkfifoat, and OFlags::PATH). This branch does not modify that executor implementation.

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[sam2tom]

Superseded by #62. PR #62 preserves the lifecycle-state semantics, includes this PR via merge commit 25e20d7, and implements the Dashboard-wide summed active Sandbox series while keeping Session metrics binary.