fix(config_edit): byte-clean writes matching Loxone's format

[tobsch]

Fixes #7.

Problem

Every write re-serialized the whole file. A raw diff on the resulting .Loxone reported ~11,500 changed lines for a single added block — the real change was drowned in formatting noise, so changes couldn't be reviewed before uploading to a live house.

The churn was purely emitter formatting that differs from Loxone's own output — the content was identical, just re-emitted differently.

Fix

Make to_bytes match Loxone's on-disk conventions so a round-trip touches only what actually changed:

1. pad_self_closing(false) — Loxone writes <C/>, xml-rs padded to <C />. This alone was ~99.5% of the diff (every self-closing element).
2. Expand attribute-less empty tags — Loxone writes <IoData></IoData>, never <IoData/>; attributed empties like <Co K="I" U="…"/> stay self-closed. (In the test config: 45 <IoData></IoData>, zero attribute-less <X/> — a consistent rule.)
3. Un-escape 
 → literal newline in attribute values (multi-line PicoC code, notification/alarm texts). Loxone keeps literal newlines and never emits 
, so this only reverses xml-rs's own escaping.
4. Trailing newline.

Result

# before: adding one block
$ diff config.Loxone out.Loxone | wc -l
11542
# after
$ diff config.Loxone out.Loxone
7716a7717,7722
> 				</C>
> 				<C Type="AMemory" … Title="__difftest__" …>
> 					<Co K="Input" …/>
> 					…

A diff of exactly the added block (+6/-0) — raw .Loxone diffs are now reviewable.

• All 78 config_edit tests pass; adds a formatting round-trip test.
• No semantic change to the DOM — only serialization.

🤖 Generated with Claude Code