fix(mac): a cached ETag is not an answer to "what server should I install"

[dvcdsys]

The bug

A fresh install of cix.app could reach a state where setup fails permanently:

Setup failed — cix could not set itself up again.
could not install the cix server: could not find a cix server to install: not modified

ensureRuntime resolved which server to download through the same
release.Client the background update check uses. That client replays GitHub's
ETag as If-None-Match, so GitHub answers 304 Not Modified and
ensureRuntime reported it as a fatal error.

The ETag describes GitHub's release listing — identical for every Mac in the
world — and says nothing about whether this machine has a runtime on disk.

Why it was permanent, not transient

1. First launch: onReady runs one background update check, which persists the
   ETag to ~/.cix/launcher.json.
2. Setup does not complete (cancelled, or the server refuses to start against an
   empty database) — no runtime installed, panel shows STOPPED.
3. Every later install attempt sends the cached ETag → 304 → "not modified".

~/.cix/ survives reinstalling the app, so the machine stayed unsetuppable
until a new server/v* release changed the listing.

The fix

updater.latestRuntime separates the two questions. On a 304 it uses the
listing this session already saw; when the ETag came from an earlier run (so the
body it matched is gone) it re-asks without it — on a copy of the client, so
the cached ETag survives for the cheap checks it exists for.

Update checks are unchanged: there a 304 still correctly means "no news".

Tests

Two new tests in cli/launcher/runtime_darwin_test.go, against a fake releases
API that honours If-None-Match:

• TestLatestRuntimeIgnoresAStaleETag — an ETag seeded into prefs (the released
  0.1.0 failure) still resolves a release, and neither the in-memory nor the
  persisted ETag is spent.
• TestLatestRuntimeReusesTheListingThisSessionSaw — a 304 after a check in the
  same process is answered from memory, with no extra request.

go build ./... and go test ./launcher/ ./internal/release/ pass.

Workaround for anyone already stuck on 0.1.0

rm ~/.cix/launcher.json && killall cix

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