Resolve workbook relationship namespace prefixes

[colin-kiegel]

Summary

Fixes #366.

Resolve workbook relationship elements by the package-relationships namespace URI and the local name Relationship, rather than matching the raw XML spelling. This allows arbitrary namespace prefixes such as ns0: and pkg: without accepting same-named elements from unrelated namespaces.

The production change is confined to src/reader/xlsx/workbook_rels.rs; it uses the existing quick-xml dependency's NsReader. Attribute reading, target normalization and pivot-cache handling remain unchanged. No dependencies or public APIs change.

Regression coverage

• Default namespace and two arbitrary prefixes.
• Foreign namespaces, an explicitly empty namespace and unrelated element names are not interpreted as package relationships.
• Namespace declarations and prefix rebinding on individual child elements are resolved in scope.
• Three synthetic workbooks, generated in memory by the existing writer, preserve text, numeric cells, a bold font and a thin border during read and write/reopen. The variants differ only in the workbook-relationships XML prefix spelling; no binary fixtures or customer data are included.

On the unchanged upstream base (4195480ede258a27eebdd202ede0e4457aff8822, version 3.1.1), the default-namespace integration case passes and both prefixed cases fail with an empty A1 instead of the expected text. All three pass with this patch.

Verification

• cargo test --locked --offline: passes (340 tests, including 79 doctests; 23 existing ignored tests).
• cargo clippy --locked --offline -- -D warnings: passes (the upstream CI command).
• cargo clippy --locked --offline --test workbook_relationship_namespaces -- -D warnings: passes.
• cargo +nightly-2026-09-11 fmt --all --check: passes.
• git diff --check: passes.

An additional cargo clippy --locked --offline --all-targets --all-features -- -D warnings run reports five existing test-code findings: four float_cmp findings in src/helper/color.rs and one uninlined_format_args finding in src/structs/pivot_table_definition.rs. The same five findings reproduce on the unchanged upstream base with the same toolchain and lockfile; they are not changed by this PR.

Tests and clippy run with Rust 1.98.1 in a disposable, non-root, network-isolated container. Formatting uses a date-pinned nightly to honor the repository's rustfmt configuration. The generated local lockfile is not committed, consistent with the repository policy.

Scope and disclosure

This is an AI-assisted fix and test implementation (gpt-6-astra high). The reproducer is fully synthetic.

This PR deliberately preserves the reader's existing self-closing-element handling; paired start/end relationship tags, other package XML readers and XML encoding support are outside this prefix-resolution fix.

[MathNya]

@colin-kiegel
Thank you for the PR.
To be honest, I hadn't really given much thought to namespaces.
I think the source code you revised is an ideal approach to solving the problem.
I'll go ahead and merge it.