Uspex Z15 SD installer (2.0)

Z15 edition of the Uspex custom firmware, version 2.0. Same platform line
as 1.4beta26 (900 MHz PLL grid, per-chip frequency control, cloud pairing)
with the 2.0 feature set ported from the Z11/Z15e builds.

2.0 highlights:
- Z11 fan model. ONE fan regulator drives the fans: Auto holds the hottest
  chip at 78 C; TT mode holds the user target (60-95 C); Manual is an honest
  percent through the measured Z15 duty->rpm map. Anti-hunting: a step is
  applied only after two consecutive ticks agree, deadband +/-2 C, climb
  +4% / descend -2%, straight to 100% at 84 C. The board-temperature curve
  (60C=37, 70C=47, 80C=59, 85C=83, 90C=95) stays as the hard floor under
  every mode. After boot the fans glide from the failsafe 91% down to the
  curve floor over 3 minutes (no hold phase). Restarts are bumpless and
  fan changes never restart the miner (register poke delivery).
- The new Fans web page holds all fan control (Auto / TT / Manual).
- The new Password page changes the web (digest) and root passwords in one
  step, as its own menu item.
- Target scan (Z11 rules): per-board frequency walks the PLL grid
  (from 750 MHz) in one-hour windows; a point is confirmed only by a REPEAT
  window at the same frequency; an unconfirmed repeat climbs HIGHER;
  LAST_GOOD is updated only with proven points, so an interrupted scan
  rolls back to a proven point. After the frequency stage the per-board
  ISL voltage walks DOWN from the stock 780 mV (5 mV steps; a >=3% hashrate
  drop in both windows versus the ratcheting best = step back to the
  previous voltage). Cloud scan target is fixed at 420 kSol total
  (140 per board).
- Fast pages: one cgminer-API fetch per render with a 2 s cross-CGI cache
  and pure-shell field extraction (no fork chains).
- Per-chip frequency control (9 ASICs) and the per-board voltage page are
  kept exactly as in 1.4beta26.
- Cloud telemetry now reports the per-board ISL voltage (chain volt fields)
  for the dashboard.

Profile ladder (cards in this order):
  Low         700 MHz / 742 mV
  Mid power   750 MHz / 750 mV
  Stock       800 MHz / 780 mV
  Target scan  (appears only after a correctly finished scan)
  Performance 860 MHz / 785 mV
  Overclock   900 MHz / 820 mV - REQUIRES a strong PSU and improved cooling.

Hard safety caps: PLL grid up to 900 MHz, board (ISL) voltage 700-850 mV.
The 95 C ladder stops mining automatically (Stock profile first, then a
full stop).

Fan behavior: fan "Fan Err!" transient low-RPM threshold raised (no false
fan-error reset at spin-up). Immersion (liquid) mode: fan thermostat off,
fans pinned at 100% as an anti-mistake safety net; the 95 C ladder still
stops mining. Factory reset and firmware flash stop any running scan and
clear scan state.

WARNING: this installer erases the user config partition (mtd2) - pools,
web password, network settings and profiles are reset to factory values.
After flashing the miner boots in the Stock profile. First boot credentials:
SSH root/admin, web root/root.

LED behavior: GREEN blinks while preparing/verifying, then ALTERNATING
red/green while the firmware is written, SOLID GREEN = completion.
SOLID RED indicates a real failure; the previous firmware stays intact.
The install log is written to Z15-INSTALL.log on the card.

1. Format a 4-16 GB microSD card as FAT32/MBR.
2. Extract every file and the bin folder to the card root.
3. Power off, insert the card and power on.
4. Watch the LEDs as described above (normally 5-10 minutes).
5. Power off, remove the card and power on normally.

This package keeps the existing NAND bootloader, environment and device tree.
It writes the primary/backup firmware ramdisks and their matching kernel slot only.
