malformed_chain poison: transient HTTP failures must not classify as chain corruption

Trap: catching a 5xx/timeout/connection-reset on chain-manifest fetch and treating it as malformed_chain looks like a safe failure-handling improvement. Fix: transient HTTP failures MUST NOT trigger a malformed_chain poison alert — that classification is reserved for genuinely unparseable/malformed bodies. User directive (Jul 25 2026): this is a permanent invariant of the chain-fetch code path. 5xx/timeout/connection-reset must fall through to the existing retry/reclassification path (chain-unavailable on persistent failure), never poison the chain marker. Confirmed in binpatch PR #7 review (Jul 27 2026): ghcrSource.resolveNightlyChain catches fetch errors per-tag setting fetchFailed=true, reports fetchFailed ? "network" : "malformed_chain" if any manifest missing. Poisoned tag's manifest fetches fine (HTTP 200) and is caught by validateNightlyChain — fetch failure classified as "network" so it never triggers a false malformed_chain poison alert.