test: cover pre-1.0 gaps and drop dead skip scaffolding

[joshua-temple]

What this change does

Targeted pre-1.0 test improvements (additive), from the coverage survey:

• source/memsource batch coverage 76.6% to 94.9% (clears the last sub-gate package): in-package tests for the batch harness lane (RunBatch/NextBatch/SettleBatch, the Batched capability path, drain).
• state/fire.go guardsPass safety branch (~33% to 88.9%): asserts a panicking guard on an eventless Always transition does not silently enable it.
• Three Example tests that double as godoc: Builder.Quench (the finalize seam), JSON-rehydrate-then-Provide-then-run, and an ActorSystem lifecycle (Spawn/deliver/Stop, previously ~0% covered).
• IR canonical-form invariant test: ToJSON(WithoutSrcPos) is byte-idempotent across a re-decode (the property evolution.DiffMachines relies on).
• Removes 17 dead t.Skip red-phase scaffolding blocks (gated on a safeBuild condition that always passes now) plus the unused helper.

Checklist

• Commits signed off (DCO) and conventional
• state tests + vet green; source/memsource ≥80% and race-clean; deterministic Examples with real // Output: