chore: declare dbus-next as optional peer dependency

[stoprocent]

Summary

• Adds dbus-next: ^0.10.0 under peerDependencies with peerDependenciesMeta.optional: true
• Replaces the undocumented, prose-only requirement (in README + lazy require error message) with a machine-readable version range
• No runtime behavior change — the binding still lazily requires dbus-next and falls back to a friendly install error if missing

Why

@stoprocent/bluetooth-hci-socket is in optionalDependencies because it's a native build that breaks installs on macOS/Windows. dbus-next is pure JS but Linux-only by purpose — making it a hard or optional dependency would pull a Linux-only library onto every Mac/Win install. The peer-optional pattern is the standard idiom for plug-in backends (eslint plugins, vite plugins, jest reporters, etc.) and gives us:

• Renovate/Dependabot/npm ls visibility into the supported version range
• No phantom install on platforms that don't use the D-Bus backend (npm 7+ does not auto-install optional peers)
• Same UX as today for missing-peer: the existing try/catch around require('dbus-next') still surfaces the install hint

The intent already leaked into lib/dbus/bindings.js:37 ("dbus-next is an optional peer of this Linux-only backend"); this just makes the manifest reflect that.

Test plan

• npm test — 17 suites, 654 tests pass
• npm run lint — clean
• npm install --dry-run — manifest validates
• CI green on Linux/macOS/Windows
• Verify npm ls dbus-next warns appropriately when the D-Bus backend is selected without dbus-next installed (manual, Linux)

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