fix(healthcheck): guard target status fields against data race

[lafoush]

Problem

monitorTarget / performHealthCheck mutate a Target's Status, LastError, LastCheck, CheckCount, and consecutive-success/failure counters from the target's monitor goroutine. Meanwhile getAllTargetsUnsafe (called by GetTargets, used for status reporting to the server) copied the whole struct with targetCopy := *target.

The monitor mutex protects the targets map, not the contents of each Target, so these accesses overlap without synchronization. The race detector flags it on any concurrent monitor + GetTargets workload — e.g. the reconnect path, where the status reporter snapshots targets while their monitor goroutines are updating status.

WARNING: DATA RACE
Read at ... by goroutine N:
  healthcheck.(*Monitor).getAllTargetsUnsafe()   // targetCopy := *target
Previous write at ... by goroutine M:
  healthcheck.(*Monitor).monitorTarget()         // target.timer / target.Status

Fix

• Add a per-Target sync.Mutex guarding the mutable status fields.
• Replace the whole-struct copy with a snapshot() taken under that lock, copying only the reported fields (Config, Status, LastCheck, LastError, CheckCount). Runtime-only fields (timer/ctx/cancel/client) are intentionally omitted — callers of GetTargets only read the reported fields, and this also avoids copying a sync.Mutex.
• The lock is not held across the health-check network request, so status snapshots are never blocked for the check timeout.
• DisableTarget's Status write is guarded too.

Test

Adds healthcheck_test.go with a regression test that exercises concurrent monitoring + snapshotting. It fails under -race without the fix and passes with it:

# without the fix
--- FAIL: TestConcurrentStatusSnapshotNoRace (race detected)
# with the fix
ok  github.com/fosrl/newt/healthcheck

go build, go vet ./healthcheck/, and gofmt are clean.

No functional/behavioral change to health checking — this is purely a concurrency-safety fix.