fix(health): measure heap pressure against the V8 heap limit (#1409)

The memory alert divided heapUsed by heapTotal. Node sizes heapTotal
just above heapUsed and grows it on demand, so a normal process reads
90% or more, and once RSS passed the 512 MB floor the health check
raised "Memory elevated" or "critically high" with nothing wrong.

The health snapshot now records v8 heap_size_limit and the thresholds
divide by it, falling back to heapTotal for snapshots written before
this change. The viewer's Heap gauge shows used against the limit, and
the alert wording says "of the heap limit".

Fixes #1406