Add read-only Netdata loopback client

[Mjones13]

Summary

Add a source-only read client for Netdata v3 over an explicitly configured loopback origin. It constructs fixed CPU and available-memory requests, validates responses against the parser in #4, and passes partial or unavailable observations through the existing model. The default transport uses an ephemeral session without cookies, credentials, or cache and rejects HTTP redirects.

This is a child of #4, which is itself a child of #2. No settings, polling, app UI, private tunnel, Mac deployment, or owner-visible telemetry is added.

Verification

• git diff --check passed on the source commit and test assertion revision 1849a8cb0a4235b4e406036cc65316e99f53c0ba.
• Added synthetic Linux tests for request construction, origin validation, endpoint failures, cancellation, and redirect policy. The first Linux ARM64 run compiled both new files but crashed in a Swift Testing optional/ternary assertion; the latest revision makes those assertions explicit. Fresh CI is pending.
• Swift, SwiftFormat, and SwiftLint are unavailable on this VPS. Native macOS validation is not claimed.
• The parser limit is enforced after transport buffering, so it is not an in-flight download limit.

Independent review finding

NETDATA-CLIENT-01 — P2, reviewed at a7d5e612c86aa0fca23637380554773995776ded: fetch(now:) classifies final sample age against the time captured before two sequential requests. A slow fetch can misclassify freshness. The original freshness-correctness claim remains open under the repository standing P2–P4 deferral; no timing repair was made. The test-only assertion revision does not address this finding.

Dependency and remaining gates

Base: #4 at 5386ee934d0d1e5291eceacc9362c2d938729d56. #4 depends on #2. The exact private Mac tunnel, expected GUID configuration, app refresh/cache injection, Systems rendering, compact menu display, and native Mac acceptance remain separate work. A failed endpoint is represented as partial or unavailable; the current host-observation model does not retain individual failure reasons.