fix(pixii): skip optional meter energy scale factor when absent

[tommylindgren]

Some PowerShaper firmware returns Illegal Data Address on register
40288. Probe once during driver_init and avoid polling it so FTW's
fresh Modbus read evidence does not fail the whole driver.

Fixes #15

Summary

Pixii catalog driver (1.1.1) and FTW v2 package target (1.2.2) now probe
meter-energy scale factor register 40288 once during driver_init and skip it
during driver_poll when absent. This prevents FTW's poll-scoped Modbus
evidence from failing the whole driver when one optional register is missing.

Driver evidence

• Driver id: pixii
• Device make/model: Pixii PowerShaper (two units on the same site)
• Firmware tested:
  • Affected unit: CPU1 v2.0.17, CPU2 v2.0.20 — register 40288 returns Modbus exception 0x02
  • Working unit: CPU1 v2.0.23, CPU2 v2.0.23 — register 40288 present, value 0 (no scaling)
• Protocol/register source: SunSpec model 213 offset 53 (meter_energy_sf, absolute 40288); Pixii Modbus mapping doc 13300
• Hardware test performed:
  • Direct Modbus TCP probe (FC 0x03) of all 37 poll registers with FTW stopped: affected unit 36/37 OK, working unit 37/37 OK
  • Live FTW site test (v1.10.5 dev, bundled drivers/pixii.lua 1.2.0 with same init-probe workaround): pixii-1 recovered from driver_poll: 1 of 37 modbus reads failed / offline to status: ok, consecutive_errors: 0
  • make test-driver ID=pixii, make package-driver ID=pixii TARGET=ftw-core, make check — all passed locally
• Known limits:
  • Import/export Wh on firmware without 40288 use sf=0; on tested working unit 40288 also reads 0, so behaviour matches
  • Primary site metering was from a separate DSMR driver, not Pixii; dispatch-critical meter_w and phase readings are unaffected
  • Older firmware (v2.0.17/v2.0.20) appears to omit 40288; v2.0.23 exposes it

Safety

• The driver starts read-only, or this is a separately reviewed control change.
• Vendor signs are converted at the driver boundary.
• Cached telemetry becomes stale instead of being re-emitted as fresh.
• No credentials, serial numbers, private addresses or site data are included.

Package or promotion evidence

• Public source SHA: (fill at merge time / from PR head commit)
• Package ID and new version: com.sourceful.driver.pixii 1.2.2
• Target matrix: ftw-core (catalog 1.1.1 + package candidate 1.2.2)
• Material and candidate artifact SHA-256: from make package-driver ID=pixii TARGET=ftw-core output at commit time
• HIL evidence or required: not applicable for this read-path bugfix; existing package HIL gates unchanged
• Control enabled: false
• Control changes include default-mode, lease-expiry and HIL evidence. (not applicable — no control changes)

Checks

• Commits include Signed-off-by.
• make test-driver ID=pixii
• make package-driver ID=pixii TARGET=ftw-core
• make check