PTP: drop the "Preferred TimeTransmitter" role

It behaved exactly like auto (clock class 248); the only difference was a
priority1 suggestion in the page. Roles are now slave|auto; to prefer this
device, use auto with a low priority1. A stored "master" is converted to
auto at boot (new cfg_set_string, like cfg_set_number).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/aes67-core-base.md

@@ -13,12 +13,12 @@ Reference devices for UI and defaults: Riedel Bolero (PTP status), Riedel Direct
 - Status table values can be text or {cls: ok|warn|bad, text} for colour.
 - Device config is merged over defaults on load, so older firmware missing keys still renders.
 - Save bumps `aes67.session_ver` (SDP o= version). Save is blocked while any field is invalid (e.g. multicast out of range).
-- UI uses IEEE 1588-2019 terms (TimeTransmitter / TimeReceiver), like Riedel. Config/API values stay master/slave/auto.
+- UI uses IEEE 1588-2019 terms (TimeTransmitter / TimeReceiver), like Riedel. Config/API values stay slave/auto.
 - AES67 fields carry Riedel Director-style "Default / Range" hints.
 - PTP preset buttons: "Riedel SIC defaults" and "AES67 media profile defaults" fill the form (Save to apply).
 
 ## Config schema (core groups)
-- ptp: {mode: multicast|hybrid, role: slave|auto|master, domain, priority1, priority2, log_sync, log_announce, announce_timeout, log_delay_req, dscp} (hardware timestamps always; not configurable)
+- ptp: {mode: multicast|hybrid, role: slave|auto, domain, priority1, priority2, log_sync, log_announce, announce_timeout, log_delay_req, dscp} (hardware timestamps always; not configurable)
 - aes67: {name, enabled, discovery: manual|sap, mcast, port, ttl, dscp, channels, mono_sum, encoding: L16|L24, rate: 48000 only (all sources are 48 kHz), ptime, pt, ssrc, clk_offset, session_id, session_ver}
 - net: {hostname, vlan, web_on: both|inet|aes67, dhcp, ip, mask, gw, dns}   (AES67 / untagged interface)
 - inet: {vlan_id, pcp, dhcp, ip, mask, gw, dns}                            (internet / tagged, only if net.vlan)
@@ -52,7 +52,7 @@ Reference devices for UI and defaults: Riedel Bolero (PTP status), Riedel Direct
 - Roles / BMCA dataset:
   - slave (TimeReceiver only): clockClass 255, never transmits time.
   - auto (default): clockClass 248, priority1 250, priority2 250. Loses to any real GM (Riedel default 128); takes over only if none.
-  - master (preferred TimeTransmitter): clockClass 248, priority1 100 (UI suggestion, editable).
+  - To prefer this device as TimeTransmitter, use auto with a lower priority1 (e.g. 100). (A separate "master" role was dropped: it behaved like auto; a stored "master" is converted to auto at boot.)
   - clockAccuracy 0xFE (unknown), offsetScaledLogVariance 0xFFFF, timeSource 0xA0 (internal oscillator).
   - clockIdentity = EUI-64 from MAC (xx-xx-xx-FF-FE-xx-xx-xx).
   - Note: Riedel Bolero reports its own clock class as 228.

docs/user-guide.md

@@ -123,7 +123,7 @@ The board locks its media clock to the PTP grandmaster on the network (IEEE 1588
 | Setting | Default | Notes |
 |---|---|---|
 | Mode | multicast | **hybrid** sends the delay requests unicast (fewer multicast packets). |
-| Role | Auto | **TimeReceiver only**: never becomes grandmaster. **Auto**: becomes grandmaster only if there is none (fallback). **Preferred TimeTransmitter**: tries to be grandmaster. |
+| Role | Auto | **TimeReceiver only**: never becomes grandmaster. **Auto**: becomes grandmaster only if there is none (fallback). To make this board the preferred grandmaster, keep *Auto* and set a low *Priority 1* (e.g. 100). |
 | Domain | 0 | Must match your grandmaster. |
 | Priority 1 / 2 | 250 / 250 | Lower wins the grandmaster election. |
 | Intervals, timeout, DSCP (46) | | Presets below set them. |