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fix: default NPDU hop count to 0xff so routed requests are not discarded (#87)

## Summary

Fixes #86.

Most service methods call `npdu.encode()` without the `hopCount`
argument, so when the receiver address contains a routed destination
(`net`/`adr` set) the NPDU hop count byte is encoded as `0x00`. Per
ASHRAE 135 §6.2.2 a router must discard routed NPDUs with hop count 0,
so services like `subscribeCov`, `subscribeProperty`,
`writePropertyMultiple`, `readRange` and `timeSync` never reach devices
behind a router and time out.
`readProperty`/`writeProperty`/`readPropertyMultiple`/`whoIs` are
unaffected because they pass `DEFAULT_HOP_COUNT` explicitly.

## Change

Default the `hopCount` parameter in `npdu.encode()` to `0xff` (and drop
the `hopCount || 0` fallback), fixing all call sites in one change. This
matches the spec's guidance to initialize the hop count to 255. Callers
that pass an explicit hop count (e.g. `DEFAULT_HOP_COUNT`) are
unchanged.

## Verification

- `npm run build` clean
- `npm run test:unit` — 204/204 pass
- Encoding check: `npdu.encode(buf, 0x04, { net: 2, adr: [12] })` now
produces `01 24 00 02 01 0c ff` (previously `...0c 00`)
- Tested end-to-end against a bacnet-stack `router-mstp` (BACnet/IP net
1 ↔ MS/TP net 2): `subscribeCov` to an MS/TP device previously logged
`NPDU: DNET=2. Discarded!` on the router and timed out; with this fix
the subscription is acknowledged and COV notifications are received.

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