fix(proxy): fence clean-close bridge replacements

[hansu650]

Fixes #1695

Summary

• advance the durable owner epoch when a fresh local HTTP bridge session replaces an unrepresented generation still owned by this instance
• fence the retiring generation's late durable release so it cannot clear the replacement owner
• keep epoch advancement separate from takeover authorization, so a newly active remote owner remains protected
• preserve ordinary in-process session reuse and cross-instance owner checks
• add deterministic product-path, call-layer, and durable-repository regressions

Verification

• regression fails deterministically with 409 bridge_instance_mismatch before the fix
• clean-close regression passes 20 consecutive runs after the fix
• related HTTP bridge and durable-session suites: 183 passed
• ruff check and ruff format --check on changed Python files
• targeted ty check on changed Python files
• OpenSpec delta added under fence-clean-close-local-replacement

The OpenSpec CLI is not available in the local Python-only task environment, so strict OpenSpec validation is left to the repository CI.

[hansu650]

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