e67a676df9af5ece5307438ae1b4ddb0730e3482 fix: uptime RPC returns 0 on first call (Lőrinc)
Pull request description:
### Problem
#34328 switched uptime to use monotonic time, but `g_startup_time` was a function-local static in `GetUptime()`, meaning it was initialized on first call rather than at program start.
This caused the first uptime RPC to always return 0.
### Fix
Move `g_startup_time` to namespace scope so it initializes at program start, ensuring the first `uptime()` call returns actual elapsed time.
### Reproducer
Revert the fix and run the test or alternatively:
```bash
cmake -B build && cmake --build build --target bitcoind bitcoin-cli -j$(nproc)
./build/bin/bitcoind -regtest -daemon
sleep 10
./build/bin/bitcoin-cli -regtest uptime
./build/bin/bitcoin-cli -regtest stop
```
<details>
<summary>Before (uptime is initialized on first call)</summary>
```bash
Bitcoin Core starting
0
Bitcoin Core stopping
```
</details>
<details>
<summary>After (first uptime call is in-line with sleep)</summary>
```bash
Bitcoin Core starting
10
Bitcoin Core stopping
```
</details>
----
Fixes#34423, added reporter as coauthor.
ACKs for top commit:
maflcko:
lgtm ACK e67a676df9af5ece5307438ae1b4ddb0730e3482
optout21:
crACK e67a676df9af5ece5307438ae1b4ddb0730e3482
carloantinarella:
Tested ACK e67a676df9af5ece5307438ae1b4ddb0730e3482
achow101:
ACK e67a676df9af5ece5307438ae1b4ddb0730e3482
musaHaruna:
Tested ACK [e67a676](e67a676df9)
Tree-SHA512: b156f7ed15c3fbb50e8a15f6c9a0d4e2ffb956d0c6ef949e0f8a661a564a20c0d3ed2149fae75ce7e2baa9326788d5037e726e7a7ac2c6ef4e70e4862cd5763f
9ec1ae0e98