Fix Windows drive letter normalization in hostless file: URLs

[lenamonj]

Fixes #889.

Url::parse("file:///C|/hello/world") returns file:///C|/hello/world. Per path state the | has to be rewritten: "If url's scheme is 'file', url's path is empty, and buffer is a Windows drive letter, then replace the second code point in buffer with U+003A (:)". The expected output is file:///C:/hello/world.

The repo already records the same defect from the other direction: <file:///w|/m> sits in url/tests/expected_failures.txt, and the WPT entry for it expects file:///w:/m.

Cause

parse_path implements the spec's "url's path is empty" test as segment_start == path_start + 1, assuming path[0] begins one byte after path_start. That holds when the serialization carries a single slash there, but a file: URL can end up with a run of them: for file:///w|/m, parse_path_start finds file:// already ending in / and returns without adding one, so parse_path appends its own.

Instrumenting the check on main shows it directly:

input file:///w|/m
  serialization="file:////"    path_start=7 segment_start=8 segment=""   -> test passes
  serialization="file:////w|/" path_start=7 segment_start=9 segment="w|" -> test fails

The only segment the test accepts is the empty one; the drive letter arrives a slash later and is never rewritten. The spellings that do work today (file:/C|/a, file://host/C|/a) carry exactly one slash at path_start, so the test holds there; the failing shapes are the ones that put a run of slashes at path_start, with the hostless file:/// form the common case.

Fix

Derive where path[0] actually starts instead of assuming path_start + 1.

Verification

Removing <file:///w|/m> from url/tests/expected_failures.txt is the regression test: the harness grades both directions, so the line fails the suite without the fix and fails it again as an unexpected success if left in place with the fix.

On main with only that line removed, cargo test -p url --test url_wpt fails:

<file:///w|/m> ... ❌
  err:  expected href "file:///w:/m", but got "file:///w|/m"

With this change it passes, and the case from #889 returns file:///C:/hello/world. Also green on x86_64-pc-windows-msvc, rustc 1.90.0: cargo fmt --all --check, cargo clippy --workspace --all-targets -- -D warnings, cargo test, cargo test --features "url/serde,url/expose_internals", cargo test --no-default-features --features=alloc.

Behaviour before and after over a grid of 410 file: inputs and join pairs: the only differences are drive letters spelled | normalized to : when they form the first path segment. The neighbouring WPT cases file:///w|m and file:///w||m, which must keep the |, are unchanged, as are file:/C|/a and file://host/C|/a.