# Fix panic when parsing with a multi-byte delimiter

## Problem

Parsing input panics when a configured delimiter is a multi-byte UTF-8
character and a line uses that delimiter:

```rust
use configparser::ini::{Ini, IniDefault};

let mut defaults = IniDefault::default();
defaults.delimiters = vec!['\u{00a7}']; // section sign, 2 bytes in UTF-8

let mut config = Ini::new_from_defaults(defaults);
config.read(String::from("[s]\nkey\u{00a7}value\n")).unwrap();
// thread panicked: start byte index 4 is not a char boundary;
// it is inside '§' (bytes 3..5 of string)
```

`delimiters` is a public, user-configurable field, so this is reachable through
the public API with safe input.

## Root cause

In `Ini::parse` (`src/ini.rs`), the key/value split computes the delimiter
position with `str::find`, which returns a byte offset, and then slices the
value starting one byte later:

```rust
let value = trimmed[delimiter + 1..].trim().to_owned();
```

`delimiter + 1` assumes the delimiter occupies a single byte. When the
delimiter is a multi-byte character, `delimiter + 1` lands inside that
character, and slicing a `str` on a non-char-boundary index panics. ASCII
delimiters such as the default `=` and `:` happen to be one byte, which is why
this went unnoticed.

## Fix

Advance past the delimiter by its actual UTF-8 length instead of a hard-coded
`1`:

```rust
let delimiter_len = trimmed[delimiter..]
    .chars()
    .next()
    .map_or(1, char::len_utf8);
let value = trimmed[delimiter + delimiter_len..].trim().to_owned();
```

For single-byte delimiters this is identical to the previous behaviour.

## Test

Added `multibyte_delimiter_does_not_panic` to `tests/test.rs`, which configures
a two-byte delimiter (`\u{00a7}`), parses `key\u{00a7}value`, and asserts the
value is read back correctly instead of panicking.

## Gate

- `cargo test --all-features` -> ok (all suites pass, exit 0)
- `cargo build --all-features` -> ok (exit 0)
- `cargo fmt --check` -> ok (exit 0)

## Counterfactual

Reverting to `trimmed[delimiter + 1..]` makes the new test panic:

```
thread 'multibyte_delimiter_does_not_panic' panicked at src/ini.rs:1020:48:
start byte index 4 is not a char boundary; it is inside '§' (bytes 3..5 of string)
```

## Flags for the account owner

`cargo clippy --all-targets --all-features -- -D warnings` currently fails, but
this is pre-existing and unrelated to this change: on an unmodified checkout the
same lint (`redundant reference in format! argument`, emitted by a newer Clippy
than the code was written against) already produces the same errors, none of
which are in the code touched here.