Document E2E test fixture register layouts

[groupsky]

Description

Create documentation explaining the register layout of each E2E test fixture to improve test maintainability.

Context

The E2E tests use emulator fixtures (port1-single-device.json, port2-multi-device.json) but the exact register addresses and values in each fixture are not clearly documented. This makes it difficult for test maintainers to understand:

• Which register addresses exist in each fixture
• What values are stored at each address
• Which register types are available (holding, input, coil, discrete)
• Expected hex representations of values

Current Issues

From PR #366 review:

• Test comment originally said "has holding registers 0-43" but fixture has sparse layout (0-10, 42-43)
• Input register availability was unclear ("may not exist" → verified: doesn't exist)
• Hex value 08FC lacked context (register 0 = voltage = 2300 decimal)

Proposed Documentation

Create tests/e2e/fixtures/README.md with:

For Each Fixture:

• Description - What device(s) it emulates
• Port assignment - Which virtual ports it uses
• Slave IDs - Which slave IDs respond
• Register layout - Table showing:
  • Address
  • Register type (holding/input/coil/discrete)
  • Value (decimal and hex)
  • Field name (voltage, current, etc.)

Example Format:

## port1-single-device.json

**Device:** EX9EM energy meter  
**Port:** /tmp/ttyV0 (tests use /tmp/ttyV1)  
**Slave ID:** 1  

### Holding Registers:
| Address | Value (Dec) | Value (Hex) | Field Name |
|---------|-------------|-------------|------------|
| 0       | 2300        | 08FC        | Voltage    |
| 1       | 520         | 0208        | Current    |
| ...     | ...         | ...         | ...        |
| 42      | 4           | 0004        | Config     |
| 43      | 1           | 0001        | Config     |

**Note:** Registers 11-41 do not exist (sparse layout).

### Input Registers:
None

### Coils:
None

### Discrete Inputs:
None

Acceptance Criteria

• README.md created in tests/e2e/fixtures/
• All emulator fixtures documented
• Register addresses, values, and types listed
• Sparse layouts clearly noted
• Hex values included for verification

Related

• PR test(e2e): add device-profiler register scanning tests #366 (identified documentation need)
• Issue Add device-profiler E2E tests #239 (original E2E test request)