feat: add TUN-less relay mode

[tionis]

This implements #44 making it easier to set up simple relay nodes.

[Copilot]

Pull request overview

This PR implements “TUN-less” operation for relay nodes so Nylon can run in environments (e.g., containers) where creating a host TUN interface is unnecessary or undesirable, aligning with issue #44.

Changes:

• Adds no_tun configuration support, including validation to prevent incompatible combinations (e.g., use_system_routing).
• Introduces a userspace-only dummy TUN device and wires it into both physical and virtual WireGuard device creation paths.
• Updates docs, sample config, and integration coverage to exercise relaying through a TUN-less node.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
state/validation.go	Rejects no_tun when combined with use_system_routing.
state/validation_test.go	Adds unit test coverage for the new validation rule.
state/config.go	Adds NoTun (no_tun) to LocalCfg.
polyamide/tun/dummy.go	Implements a dummy TUN device that blocks reads until closed and discards writes.
polyamide/tun/dummy_test.go	Adds tests for dummy device semantics (name/MTU/write/close behavior).
core/sys_virtual.go	Uses dummy TUN when NoTun is enabled in virtual mode.
core/sys_physical.go	Uses dummy TUN when NoTun is enabled in physical mode and avoids darwin utun naming in that case.
core/nylon_wireguard.go	Skips alias/interface init and system sync when NoTun is enabled.
integration/routing_test.go	Adds an integration test for routing through a TUN-less relay.
example/sample-node.yaml	Documents no_tun and fixes post_down indentation.
docs/reference/config.mdx	Documents no_tun and its constraint with use_system_routing.

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