fix(engine-manager): report install steps without fake percentages - #145

Summary

Engine installs sat at a hardcoded 75% for the whole install step, and verified was hardcoded at 50%, so the bar went 100% → 50% → 75%. The install step runs opaque commands (tar/unzip for Ollama, LM Studio's vendor installer) with no streamed output, so real progress isn't measurable. This PR stops inventing numbers: those steps are now reported as indeterminate, and the UI shows just the stage ("Installing").

• nvpair-engine-manager: emitInstallProgress omits percent from engine:install-progress when it is 0 (indeterminate), matching the existing pull-progress convention (wirePercentIncluded). verified and both installing paths now send 0. Real download byte progress and terminal frames (done/already-installed = 100, failed = -1) are unchanged. A download without Content-Length now shows "Downloading" instead of "0%".
• nvpair-tui: renders (N%) only for a measured percent (previously would show (0%), and already showed (-1%) on failure).
• Desktop: the bridge projected a missing percent through numberValue, which returns 0, so the UI would have shown "Installing · 0%". Both install projections (local applyEngineManagerProgress, remote applyRemoteEngineProgress) now map a missing or zero percent to undefined via measuredPercent, so BackendHeader and WelcomeEngineRow render the stage alone. Pull keeps its existing mergePullProgressPercent behavior.
• Failure frames: reportInstallFailed now goes through the shared install emitter, and the TUI shows the failure reason (install <engine> failed: <error>).
• Docs: spec.md / README.md mark percent optional on engine:install-progress, engine:pull-progress, and engine:remote-progress, and state the shared rule once.

Audited components

• Producer: nvpair-engine-manager (install.go, executor.go, progress.go).
• Remote relay: controlstream.go / remote.go already omit percent 0; unchanged.
• Broker: relays engine:* verbatim; unchanged.
• Consumers: desktop modular-state.ts updated (local and remote install projections); BackendHeader.tsx / WelcomeEngineRow.tsx render the stage alone once percent is undefined; model-pull UI is unaffected; nvpair-tui updated.

Testing

• go test ./... in services/nvpair-engine-manager and services/nvpair-tui
• New Go tests: indeterminate install frame omits percent; full fetch+run install emits a measured percent on download, none on verified/installing, 100 on done, and no unexpected stages
• New desktop tests: local and remote install frames drop to percent: undefined after a 100% download (verified failing without the fix)
• New TUI tests: percentSuffix and installStatus
• npm run lint, npm run typecheck, npm run dead-code:check, npm run service-contracts:check, npm run test:unit in desktop/

Changelog title

Engine install no longer shows a fake percentage

Changelog body

• Installing an engine no longer sits at a made-up 75%. Download progress still shows a real percentage; the install step itself now shows "Installing" with no number.

Bumps

• services: patch
• nvpair-cluster-manager: none
• nvpair-engine-manager: patch
• nvpair-errors: none
• nvpair-job-scheduler: none
• nvpair-manual-nodes: none
• nvpair-node-info: none
• nvpair-node-scanner: none
• nvpair-node-settings: none
• nvpair-proxy: none
• nvpair-tui: patch
• nvpair-ui-broker: none
• nvpair-workload-manager: none