Percent-decode local image hrefs - #1121

Fixes #1073.

The bug

An <image> href is an IRI, so SVG editors percent-encode characters that
can't appear in one. Inkscape, for example, writes a local file named
images/细节3-mine.png as:

<image href="images/%E7%BB%86%E8%8A%823-mine.png" .../>

ImageHrefResolver::default_string_resolver passed that string straight to
Options::get_abs_path and then to Path::exists, so it looked for a file
literally named %E7%BB%86%E8%8A%823-mine.png. That file doesn't exist, and the
image was silently dropped:

Warning (in usvg::parser::image:110): 'images/%E7%BB%86%E8%8A%823-mine.png' is not a path to an image.

Reproducer (from the issue):

repro/
  encoded.svg      <image href="images/%E7%BB%86%E8%8A%823-mine.png" .../>
  images/细节3-mine.png

resvg --resources-dir ./repro ./repro/encoded.svg out.png

The same SVG with the decoded file name in the href renders fine.

The fix

The default string resolver now percent-decodes the href and retries, using a
small local decoder (no new dependency).

The raw path is still checked first and the decoded path is only used as a
fallback, so a file whose name genuinely contains a percent sign — including
one that happens to look like a valid escape sequence, e.g. %41.svg — keeps
resolving exactly as before. Nothing that works today changes behaviour; this
only adds a second attempt for hrefs that currently fail. Invalid escape
sequences are left as-is, and if the decoded bytes aren't valid UTF-8 the
original path is kept.

This is fixed in the resolver rather than in the parser because that's where the
href is turned into a filesystem path, and it keeps custom
ImageHrefResolvers free to do their own thing.

Deliberately out of scope

• data: URLs — they're handled by resolve_data and never touch the filesystem.
• fontdb/resources_dir path handling in general.
• Interpreting the href as a full URL (scheme, authority, query, + as space).
  usvg doesn't resolve URLs, and treating + as a space would break existing
  file names.

Verification

New tests in crates/usvg/tests/parser.rs. Before the change:

running 2 tests
test percent_encoded_image_href ... FAILED
test image_href_with_literal_percent ... ok

---- percent_encoded_image_href stdout ----
thread 'percent_encoded_image_href' panicked at crates/usvg/tests/parser.rs:617:5:
assertion failed: is_image_resolved(&dir, "images/%E7%BB%86%E8%8A%823-mine.svg")

test result: FAILED. 1 passed; 1 failed; 0 ignored; 0 measured; 27 filtered out

After:

test result: ok. 29 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

cargo test --all --release is green: 1730 render tests, 29 usvg parser tests,
31 usvg writer tests, 2 resvg unit tests, 1 doctest. cargo fmt --all --check,
.github/copyright.sh and typos are clean (the two typos hits on main —
oppen in the changelog and planed in parser/filter.rs — are pre-existing
and untouched). Also built with an older toolchain to check nothing here needs a
recent compiler.