Advertise repeat capability via FEAT1 flags

[wbijen]

Summary

Implements #1906 — nodes that forward packets but aren't dedicated repeaters (room servers with repeat enabled, companions with client_repeat) can now advertise this via a bitmask in the existing FEAT1 advert field.

This helps companion apps distinguish relay-capable nodes in path displays and resolves hash collisions where a room server was being confused with a repeater (as reported in #1866 and #1707).

What changed

• Added ADVERT_CAP_REPEAT (0x0001) — first bit in the FEAT1 bitmask, meaning "this node forwards packets".
• buildAdvertData() and createSelfAdvert() gain an optional feat1 parameter (default 0, no extra bytes on wire).
• Room server sets ADVERT_CAP_REPEAT when repeat is enabled (!disable_fwd).
• Companion radio sets ADVERT_CAP_REPEAT when client_repeat is on (3 call sites: manual advert, self-export, periodic advert).
• Dedicated repeater sets ADVERT_CAP_REPEAT for consistency, even though ADV_TYPE_REPEATER already implies it.

The generic FEAT1 naming is kept intentionally — the 16-bit field carries different kinds of information (capabilities, flags, properties) depending on context, so a neutral name fits better than something specific.

Backwards compatibility

Fully backwards compatible in both directions:

• Old clients that see the ADV_FEAT1_MASK bit simply skip the 2-byte field (already wired up in the parser).
• Old firmware sends feat1 = 0, so the field is omitted from the wire — no extra bytes.

Future FEAT1 bits

The uint16_t bitmask has 15 bits remaining. Some candidates:

Bit	Idea	Rationale	Related
TBD	ADVERT_FLAG_OPEN_ROOM	Room server has no password (or uses default) — apps could show a lock/unlock icon without needing a round-trip and skip the login screen	PR #901
TBD	ADVERT_FLAG_READONLY	Room server has allow_read_only enabled — apps could indicate guest access is available

These don't have concrete firmware/app consumers yet, so they're not included — just noted for future reference.

Test plan

• Builds cleanly against heltec_v4_companion_radio_usb
• Builds cleanly against heltec_v4_room_server
• Builds cleanly against heltec_v4_repeater
• Builds cleanly against heltec_v4_sensor (unchanged, still passes feat1 = 0 via default)
• Verify on-air: companion with client_repeat on → advert includes FEAT1 field with 0x0001
• Verify on-air: room server with repeat → advert includes FEAT1 field with 0x0001
• Verify old companion app still parses adverts correctly (skips unknown 2 bytes)

[jbrazio]

Please see meshcore-dev/meshcore.js#17