refactor(client): share the registered service decision between clients

[kitlangton]

Why

When a registered service answers, both ensure() loops decide what to do with the same chain of matchesVersion / compatible / state checks. #50825 had to add the protocol guard to both copies. The rules are easier to review, and to test without child processes, as one function.

What Changes

decide(service, options) in service-probe.ts returns what to do. Each client performs it in the same order as before.

Service answered	Version requirement	decide()	Client then
no health endpoint (compatible: false)	none, matching, or accepting	fail (incompatible health protocol)	fails
no health endpoint	not met	replace, pty: "handoff"	announce, stop, evict
compatible, ready	met	reuse	completes terminal handoff, returns it
compatible, starting	met	wait	keeps polling
compatible, failed	met	fail ("Background service failed to start")	fails
compatible, any state	not met	replace, pty: "handoff" only when ready, otherwise "clear"	warns if not ready, stops, evicts

Error messages are byte-identical.

Scope

Stacked on #53240, with no behavior change intended. That completes the shared lifecycle core: the probe, startup attempts, and this decision. terminate() and stop() stay in each client because they are almost all IO.

Verification

cd packages/client
bun typecheck
bun run test test/service-decision.test.ts test/service-contender-pool.test.ts test/service.test.ts test/promise-service.test.ts test/service-reconnect.test.ts --rerun-each 3

• 180/180 across three runs of each. The black-box service tests are unchanged.
• New service-decision.test.ts covers the table above as plain unit tests.
• Mutation check: disabling the protocol guard in decide() fails fix(client): preserve service on protocol-only reconnect mismatch #50825's three reconnect tests in both the Effect and Promise suites, plus the unit test.

Stack

1. fix(client): preserve background service startup failures #50784
2. refactor(client): share the service health probe between clients #53237
3. refactor(client): share startup attempt bookkeeping between clients #53240
4. refactor(client): share the registered service decision between clients #53241 👈 current