Add donor STUN configuration and IP-based fallback pool

[myleshorton]

Adds UnboundedConfig.stun_servers and a shared IP-based fallback pool so donor clients can initialize WebRTC without downloading a STUN list from GitHub. The optional field preserves compatibility with older servers and clients; the fallback returns a fresh slice on each call.

Stacked on #32 because current API and Radiance main already pin that user-message contract. Keeping this base preserves those packages when the consumers bump their dependency. Merge #32 first, then retarget this PR to main.

Validation: go test ./...; all eight fallback servers answered live STUN binding requests during investigation. Comment review completed.

Companions: getlantern/radiance#634 and https://github.com/getlantern/lantern-cloud/pull/3324. This provides the configuration contract; it does not change the default broflake fetcher.

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[Copilot]

🔵 Needs a closer look

Address the two moderate test-coverage issues in types_test.go.

Pull request overview

Adds optional donor STUN configuration and an IP-based fallback pool for WebRTC initialization.

Changes:

• Adds UnboundedConfig.STUNServers with stun_servers JSON compatibility.
• Adds a fresh-slice default STUN server pool.
• Adds serialization and slice-isolation tests.

File summaries

File	Summary	Findings
types.go	Defines STUN configuration and fallback servers.	None
types_test.go	Tests serialization and fallback slice isolation.	Two moderate findings: assert the exact JSON key and cover nil/empty omission cases.

Review details

Suppressed comments (2)

types_test.go:279

• The round-trip assertion cannot detect a wrong or missing JSON tag: json.Marshal and json.Unmarshal would use the same incorrect key. Since stun_servers is the compatibility wire contract, assert that encoded contains the exact key (as the neighboring field tests do) so a future tag regression fails.

	encoded, err := json.Marshal(original)
	assert.NoError(t, err)
	var decoded UnboundedConfig
	assert.NoError(t, json.Unmarshal(encoded, &decoded))
	assert.Equal(t, original.STUNServers, decoded.STUNServers)

types_test.go:279

• The compatibility promise depends on omitempty omitting both a nil and a non-nil empty STUNServers slice, but this test only covers a populated list. Please add the nil/empty cases (as the neighboring slice-field tests do) so a future tag or type regression cannot make older clients emit stun_servers unexpectedly.

	encoded, err := json.Marshal(original)
	assert.NoError(t, err)
	var decoded UnboundedConfig
	assert.NoError(t, json.Unmarshal(encoded, &decoded))
	assert.Equal(t, original.STUNServers, decoded.STUNServers)

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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[myleshorton]

Addressed both test findings from the Copilot review in f825bae: the round-trip test now explicitly asserts the stun_servers JSON key, and TestDonorSTUNConfigOmission verifies both nil and non-nil empty lists serialize without that key. go test ./... -timeout=90s passes. These were review-body findings, so there are no inline threads to resolve. The companion PRs continue pinning 674a990 because this follow-up only changes tests.