Relativize LCOV paths against a root, match coverage roots at segments

[davidanthoff]

Groundwork for julia-testitems/testitem-workflow#11 (Codecov reporting 0%). The workflow-side fix is julia-testitems/testitem-workflow#13; this PR fixes the LCOV output itself.

write_lcov gains root

write_lcov emitted the absolute path uri2filepath hands back — and on Windows a backslashed one, which no LCOV consumer recognizes at all, so a Windows matrix leg contributed nothing to a merged report even before the absolute-path problem. Coverage services match SF: paths against paths in the repository, so the absolute paths of a CI runner match nothing, and a fully covered package can come out at 0%.

It now takes the same root keyword write_junit_xml already has, implemented by the same helper: _relative_path is split into _report_path (returns nothing for a URI it cannot turn into a path) plus the URI-falling-back wrapper the JUnit classnames need. So both writers relativize identically, abspath a relative root the same way, and refuse to walk out of the root with .. the same way.

• Paths always use / separators now, root or no root.
• A file outside root keeps its absolute path rather than a ..-heavy one — the choice the JUnit writer already makes, and the one that keeps a stray record looking like a stray record rather than a coverage regression.

write_lcov is itself unreleased (it is in the 2.0.0 section of the CHANGELOG), so the signature change is not breaking for anyone; the existing bullet is amended rather than a new one added.

Coverage roots match whole path segments

Separate bug in the test process, found while making TestItemApp narrow its roots to src/ext: a root is a folder URI and filepath2uri never leaves a trailing slash, so startswith also accepted siblings whose names merely started with it — a root of <workspace>/Foo collected coverage for <workspace>/Foo2.

Tests

Four new items, all passing; the whole of test_junit.jl + test_coverage.jl is green (20/20).

• LCOV relativizes against a root, including a relative root (the relpath trap the JUnit test already guards).
• A file outside the root stays absolute.
• Forward slashes everywhere, non-file: URIs skipped.
• Coverage roots match whole path segments — verified it fails on the old prefix match and passes with the fix.

Note for whoever tags this

Project.toml says 1.7.1-DEV while the CHANGELOG heads its unreleased section ## [2.0.0] - Unreleased and contains a breaking JSONRPC entry. Not touched here, but the two disagree.

🤖 Generated with Claude Code