Fix durable Tailscale device identity

[fifthsegment]

Summary

• persist linked Tailscale addresses so canonical device identity survives GateSentry restarts and temporary peer disconnects
• safely reconcile passive address-only duplicates while retaining ambiguous durable or policy-referenced conflicts
• keep policy resolution bound to stable linked aliases and show overlay addresses in the device inventory
• prevent DDNS matching and cleanup from corrupting or deleting linked device identity
• persist ordinary discovered devices when a policy is assigned so their canonical IDs and assignments survive restart and MAC-based rediscovery
• reject assignment writes when device identity cannot be retained, while clearing an assignment leaves the retained identity intact

Verification

• go test ./application/dns/discovery ./application/webserver/endpoints ./application/dns/server ./application/policy
• make verify
• git diff --check
• Svelte diagnostic baseline matched: 16 known errors and 19 known warnings, with no diagnostic changes
• 11 frontend test files passed, 45 tests total

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Codecov Report

❌ Patch coverage is 53.41615% with 75 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.34%. Comparing base (f271200) to head (25ac7a0).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
application/dns/discovery/store.go	50.00%	43 Missing and 5 partials ⚠️
application/dns/server/server.go	33.33%	10 Missing ⚠️
application/dns/discovery/types.go	0.00%	6 Missing ⚠️
application/webserver/endpoints/handler_policy.go	62.50%	4 Missing and 2 partials ⚠️
...ation/webserver/endpoints/handler_device_policy.go	70.00%	2 Missing and 1 partial ⚠️
application/dns/discovery/persistence.go	85.71%	1 Missing and 1 partial ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
+ Coverage   50.46%   51.34%   +0.87%     
==========================================
  Files         128      128              
  Lines       12253    12803     +550     
==========================================
+ Hits         6184     6574     +390     
- Misses       5308     5443     +135     
- Partials      761      786      +25     

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