feat: uninterrupted tunnel operations

[nicodes]

Status

Draft integration work; not ready for review or release. The agent still advertises protocol v4 and explicitly refuses v5 dispatch until coordinated client, backend and tunnel-handoff verification is complete.

Implemented

• Distinct v5 framing and sealed connection bindings, preserving v0-v4 contracts.
• Bounded absolute-offset output replay with explicit gaps rather than silent resets.
• Four retained input writers per PTY; replacement connections share a slot; only fully written and client-confirmed disconnected writers are reclaimable.
• Bounded scheduler integration, partial-write ordering, retry deduplication, and coalesced sealed ACKs issued only after actual PTY writes.
• Ready/replay attachment handling; failed replay preserves the PTY and existing attachment.

Local verification

Go 1.25.13 on Linux: go vet ./...; go test -race -count=1 ./...; govulncheck ./...; gofmt and git diff --check. Focused attachment tests also passed ten race-enabled repetitions. No reachable vulnerabilities reported; three advisories remain in required modules outside the analyzed reachable code.

Still required

Distributed fenced tunnel ownership, make-before-break handoff, coordinated client rendering/input resumption, bounded release retirement, and complete end-to-end compatibility/continuity verification. Local tests are not a production or universal uninterrupted-operation guarantee. No merge or deployment is authorized by this draft.