cs-test: array-shaped expected.fields and rendered.fields

[manuelwedler]

Summary

The ERC-7730 registry's v2 test schema is reshaping expected.fields from a label-keyed map to an ordered array of {label, value} entries (spec PR on branch common-test-strategy). Array-iteration descriptor paths like signers.[] (e.g. Safe setup) render multiple entries with the same label — a map collapses them; an ordered array preserves them.

This PR updates cs-test to read and emit the new shape:

• schema — Expected.fields and NestedExpected.fields become Vec<FieldEntry>. New FieldEntry { label, value } / FieldValue replace the label-keyed IndexMap<String, FieldExpected>.
• compare — positional comparator. New FieldCountMismatch / FieldLabelMismatch variants; FieldValueMismatch / FieldKindMismatch carry an index. The old label-set logic (FieldMissing, FieldExtra, AmbiguousLabel) is gone — duplicates at different positions now compare cleanly instead of being flagged as ambiguous.
• results.rs — cases[].rendered.fields emits the same array shape. Duplicate labels are preserved end-to-end instead of being silently collapsed by IndexMap::insert.
• report, fixtures, smoke.rs — track the new variants and shape.
• Drops unused indexmap dep.

Test plan

• cargo test -p cs-test (22 unit + 4 smoke, including new positional cases: duplicate-labels-pass, swapped-positions-fail, length-mismatch, label-mismatch)
• cargo clippy -p cs-test --all-targets -- -D warnings
• End-to-end against the registry's only converted fixture, aave/testsv2/calldata-lpv2.tests.json on branch common-test-strategy: all 3 cases pass; results.json emits cases[].rendered.fields as ordered [{label, value}, ...].

Out of scope

• Other ~250 testsv2 files in the registry — those land registry-side.
• Descriptor format changes — unaffected.

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