State/liveness keyed on PID alone is vulnerable to PID reuse

## Description

Dev Proxy tracks detached instances by PID alone (`state-.json`), and `StateManager` determines whether an instance is "alive" purely by checking whether a process with that PID currently exists (`StateManager.IsProcessRunning(int pid)` → `Process.GetProcessById`).

Operating systems recycle PIDs. After an instance exits (cleanly or via crash), its PID number can be reassigned to a completely unrelated process. When that happens:

- `IsProcessRunning(pid)` returns `true` for the stale state record, so Dev Proxy believes its old instance is still alive.
- Commands that key on liveness (`stop`, `status`, `LoadAllStatesAsync`, `FindSystemProxyInstanceAsync`, and the crash-recovery / orphaned-system-proxy reconciliation added in the `stop --force` fix) can act on — or refuse to act on — the wrong process.

This is a pre-existing, low-probability correctness issue in the whole detached-instance design; it is independent of any single command.

## Suggested fix

Store additional identity beyond the PID in the state file and verify it before treating a PID as "our" live instance. Options:

- Persist the process **start time** (`Process.StartTime`) alongside the PID, and in `IsProcessRunning` compare the running process's start time to the recorded value — a mismatch means the PID was reused and the record is stale.
- Optionally also persist the process name / a Dev Proxy marker as a secondary check.

`Process.StartTime` is available cross-platform in .NET and is the standard, low-cost way to disambiguate PID reuse.

## Notes

- Found while implementing the fix for #1731 (`devproxy stop --force` cannot restore the system proxy after a crashed instance). That fix relies on `asSystemProxy` state records to reconcile orphaned system-proxy registrations; PID-reuse hardening would make that reconciliation (and all liveness checks) more robust but is intentionally out of scope for that change.
- Affected code: `DevProxy/State/StateManager.cs` (`IsProcessRunning`, `LoadStateFromFileAsync`, `GetOrphanedSystemProxyStatesAsync`), `DevProxy/State/ProxyInstanceState.cs`.

Start by reading DevProxy/State/StateManager.cs, especially IsProcessRunning, LoadStateFromFileAsync, and GetOrphanedSystemProxyStatesAsync, then inspect DevProxy/State/ProxyInstanceState.cs to understand the state-file data. Trace how detached-instance commands use PID liveness. Done means stale state is rejected when a PID has been reused, while legitimate instances continue to be recognized.

Written by the indexing model from the issue text.