Decode bounded Netdata v3 host metrics

[Mjones13]

Summary

Add a bounded, pure Netdata 2.11.0 v3 JSON2 decoder for system.cpu and mem.available. It validates the expected machine GUID, context, dimensions, units, point layout, value ranges, and sample age. CPU busy excludes iowait; available memory is stored separately from used memory. The hub can accept observations keyed by an existing configured environment, while the default remains unsupported.

This is a source-only child of #2. It does not start a network collector, change settings, render a menu value, install software, or establish private Mac-to-VPS access. The parent remains the route to main.

Verification

• git diff --check passed at c63696b41bfaa89c8e3004a9c09c932fa90e701f.
• Added synthetic Linux Swift tests for schema, identity, conversion, stale/missing/partial states, and environment injection.
• Local swift test --filter ObservabilityHubLinuxTests was unavailable (exit 127: Swift is absent on this VPS); native macOS validation is also pending. This non-draft PR requests CI on the exact candidate.
• Independent read-only code review found no supported functional defect. It recorded NETDATA-DOC-01 (P3): the decoder docstring says any future sample is stale while the code permits a five-second tolerance. The finding is preserved for owner disposition under the repository's P2–P4 rule.

Dependency and remaining gates

Base: #2 at 0bdf8a262bc9342d9b62a5f12f8062db22273b50. Source CI and a verified private Mac client route are separate from the already activated VPS Netdata Agent. No working native menu or owner-visible dashboard is claimed.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}