Add ePIC firmware support for Antminers

[ankitgoswami]

This plan adds VARIANT_EPIC as a first-class firmware variant alongside vnish/braiins/luxos/marathon, and unifies native ePIC hardware under the same variant (since ePIC's stock firmware is ePIC firmware).

Changes

1. plugin/asicrs/src/capabilities.rs — variant registration

• After L23 add:

  pub const VARIANT_EPIC: &str = "epic";

• After L29 add:

  const DISPLAY_EPIC: &str = "ePIC";

• firmware_to_variant: add an epic arm that returns VARIANT_EPIC when the firmware string contains "epic" (case-insensitive). Place it before the marafw/marathon branch.
• detect_variant: add a corresponding make-fallback branch for "epic" so a make of "ePIC" resolves to VARIANT_EPIC even when the firmware string is empty.
• firmware_manufacturer: add VARIANT_EPIC => Some(DISPLAY_EPIC).
• default_credentials: add (FAMILY_ANTMINER, VARIANT_EPIC) => admin/letmein arm before the catch-all (FAMILY_ANTMINER, _) => root/root. The existing (FAMILY_EPIC, _) => admin/letmein arm already covers native ePIC hardware regardless of variant.
• Tests (in the existing #[cfg(test)] mod tests block):
  • firmware_to_variant("ePIC") == VARIANT_EPIC
  • firmware_to_variant("epic 1.2.3") == VARIANT_EPIC
  • detect_variant("AntMiner", "ePIC") == VARIANT_EPIC
  • detect_variant("ePIC", "") == VARIANT_EPIC (make fallback)
  • firmware_manufacturer(VARIANT_EPIC) == Some("ePIC")
  • default_credentials(FAMILY_ANTMINER, VARIANT_EPIC) returns admin/letmein
  • Follow AAA test style (per ~/CLAUDE.md)

Do NOT add VARIANT_EPIC to the antminer group inside make_to_family. That table is keyed by the make string, and asic-rs returns make="AntMiner" for ePIC-on-Antminer; the existing "epic" → FAMILY_EPIC mapping must stay so native ePIC hardware still resolves to FAMILY_EPIC.

2. plugin/asicrs/config.yaml — enable the variants

• Under miners.antminer, add:

      epic:
        enabled: true

• Replace the miners.epic.stock block (L35-37) with:

    epic:
      epic:
        enabled: true

  (After this change, native ePIC hardware resolves to family=epic, variant=epic — internally consistent with the firmware string asic-rs returns.)

3. plugin/antminer/internal/driver/driver.go — defensive skip

ePIC firmware speaks HTTP (not CGMiner) on port 4028, so the Go plugin's GetVersion() call should fail to even parse a response. But for symmetry with the existing vnish/braiins/luxos/marafw markers, and to handle any future ePIC firmware that might expose CGMiner-compatible version output:

• After L42 add:

  firmwareMarkerEpic = "epic"

• Append firmwareMarkerEpic to nonStockFirmwareMarkers (L47-52).

No change needed to the LUXminer-style explicit field check (L249-252) — the substring marker is sufficient.

4. Contract test fixture — tests/plugin-contract/testdata/antminer-epic/

Create a fixture mirroring antminer-vnish/ but adapted for ePIC's HTTP-only API on port 4028:

• manifest.json:

  {
    "model": "S19jPro",
    "manufacturer": "ePIC",
    "firmware": "epic",
    "firmware_version": "<recorded>",
    "plugin": "asicrs",
    "recorded_from": "<ip>",
    "recorded_at": "<timestamp>",
    "ports": { "rpc": 4028, "web": 80 }
  }

• HTTP fixtures recorded against a real ePIC device covering at minimum the endpoints asic-rs's EPicFirmware probes (asic-rs-firmwares/epic/src/firmware.rs:81,136):
  • GET http://<ip>:4028/capabilities → returns {"Model": "Antminer S19j Pro", "Model Subtype": "..."}
  • GET http://<ip>:4028/summary → returns {"Software": "ePIC v...", ...}
  • Plus whatever the asic-rs PowerPlay backend calls for telemetry/pools