Problems with `identity` in cpost configs

While trying to migrate some v2 cpost configs to sr3 cpost, I came across some problems.

### First problem:

The `identity` option isn't parsed correctly:

| config option | parsed as `sum=` | result |
|-|-|-|
| `sum d` md5 on data | `d` | ✅
| `sum n` md5 on file name | `n` | ✅
| `sum s` sha512 on data | `s` | ✅
| `identity md5name` | `m` | :x: should be `n` md5 on file name
| `identity md5` | `m` | :x: should be `d` md5 on file data
| `identity sha512` | `s` | ✅

### Second problem:

When using `sum n` (md5name) with v03 format messages, identity is missing from the message:

`{ "pubTime" : "20260129T174716.2993753", "baseUrl" : "sftp://localhost/", "relPath" : "tmp/test", "source" : "bunnymaster", "size" : "5", "atime" : "20260129T161256.87121107", "mtime" : "20260129T161246.786308", "mode" : "644" }`

compared to v02 format messages:

Maybe this was intentional, because `identity` is optional in v03 messages and md5 on the file name is kind of useless? But the behaviour is inconsistent with the Python version and surprising/confusing.

Reproduce the cpost migration cases for identity, sum n, and v03 versus v02 messages, then trace the configuration parsing and message-generation entry points. Compare the behavior with the Python version; done means md5name and md5 map to the reported sum modes and v03 messages consistently include identity when sum n is selected.

Written by the indexing model from the issue text.