[FR] cardano-testnet: add a status command

[palas]

Part of #6687 (subtask 2 of 2). Depends on the manifest sub-issue.

Problem

There is no way to ask a running testnet "are you alive?". A dead node is silent (nothing
monitors the child processes after startup), and a chain stall (#5762) is only visible by
reading logs. Consumers write their own polling loops.

Proposal

cardano-testnet status [--output-dir DIR] [--json] [--timeout SECONDS]

Same --output-dir flag and ./testnet default as the cardano command. Read-only, always
terminates (per-node timeout, default 5s). It reads manifest.json, then asks every node
for its chain tip over its socket (getLocalChainTip — the same lightweight query the
chain watchdog polls with), and reports:

Per node: ok (fresh tip) / stalled (answers, no fresh tip) / unreachable (no answer,
process alive or unknown) / down (no answer, process known dead). The PID check is only a
hint; the socket answer decides.

A tip is "fresh" when younger than max(60s, 2 x forecast horizon) — the same threshold the
chain watchdog already uses for "chain dead" (60s at default settings; status applies it to
the tip's age in one snapshot). One shared number, no new magic value. A node answering
"no blocks yet" (tip at genesis) counts as stalled.

Network state and exit code (answers "what if one node is unhealthy?"):

state	rule	exit
RUNNING	every node ok	0
DEGRADED	chain producing (>= 1 node ok), some node not	0
STALLED	nodes answer, no fresh tip anywhere	3
STOPPED	no node answers	4
NO-TESTNET	no manifest.json in the directory (+ hint)	5

Exit 0 = "the chain is producing blocks" (the rule from #6687), so
cardano-testnet status && run-tests works, even when a relay died. Exit 1 stays what it
already means everywhere: the command itself failed. Strict "all healthy" checks use the
JSON field status == "running".

--json prints one stable object (schemaVersion'd like the manifest, schema in the repo):
overall status, best tip (slot/block/hash/age), and the per-node list with tips and ages.
Polling status until exit 0 is the intended "wait until ready" loop (NO-TESTNET covers
the not-started-yet window; the manifest only appears once the network is ready).