fix(backend): clean stale forward runtimes on entry updates (#307)

Entire-Checkpoint: 6a6b91fb5f0c

plans/029-issue-281-contract-repro.md

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+# 029 - Issue 281 Contract Repro
+
+## Goal
+Add a contract test that reproduces issue #281: after changing a tunnel's entry node, forward runtime cleanup does not remove the stale service from the old entry node.
+
+## Checklist
+- [x] Review existing contract test helpers for mock node command recording.
+- [x] Add a contract test that updates a tunnel entry node while a forward is bound to the tunnel.
+- [x] Assert the new entry node receives forward sync commands and the old entry node does not receive forward cleanup, reproducing the bug.
+- [x] Run the focused contract test and capture the failure.
+
+## Test Record
+- Command: `cd go-backend && go test ./tests/contract/... -run TestTunnelUpdateChangesEntryNodeButLeavesOldForwardRuntimeContract`
+- Result: failed as expected with `expected old entry node to receive forward DeleteService cleanup for 1_2_281, got none`.

plans/030-fix-issue-281-stale-forward-runtime-cleanup.md

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+# 030 - Fix Issue 281 Stale Forward Runtime Cleanup
+
+## Goal
+When a tunnel's entry nodes change, remove forward runtime services from entry nodes that are no longer part of the tunnel before syncing the forward to its new entry nodes.
+
+## Checklist
+- [x] Review the tunnel update flow and identify where old/new entry node sets are available.
+- [x] Add backend cleanup for forward runtimes on removed entry nodes.
+- [x] Keep existing forward port rebuild and forward resync behavior intact.
+- [x] Run focused contract regression tests for the issue 281 repro.
+
+## Test Record
+- Command: `cd go-backend && go test ./tests/contract/... -run 'TestTunnelUpdateChangesEntryNodeButLeavesOldForwardRuntimeContract|TestTunnelUpdateRecoversFromAddressInUseContract'`
+- Result: passed.

plans/031-entry-transition-regression-coverage.md

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+# 031 - Entry Transition Regression Coverage
+
+## Goal
+Expand issue #281 regression coverage to verify forward runtime cleanup and `forward_port` rebuilding across both single-entry to multi-entry and multi-entry to single-entry tunnel updates.
+
+## Checklist
+- [x] Review the current issue 281 contract repro and reuse its mock-node recording helpers.
+- [x] Add a broader contract test that exercises both entry transition directions.
+- [x] Assert removed entry nodes receive forward cleanup and retained/new entry nodes receive forward sync.
+- [x] Run focused contract tests and record the result.
+
+## Test Record
+- Command: `cd go-backend && go test ./tests/contract/... -run 'TestTunnelUpdateChangesEntryNodeButLeavesOldForwardRuntimeContract|TestTunnelUpdateEntryTransitionsCleanupForwardRuntimeContract|TestTunnelUpdateRecoversFromAddressInUseContract'`
+- Result: passed.