bug: make array-kind validation case-sensitive under nocasematch

[codeforester]

Goal

Make array-kind validation independent of the caller's nocasematch shell option.

Background

PR #521 (fix: validate caller-owned output attributes) added caller-owned output
contracts and a regression test that enables shopt -s nocasematch. The hosted
validation run for that PR fails on both Ubuntu and macOS in the same test:

array output contracts distinguish indexed from associative arrays with nocasematch

The validator currently classifies the kind marker with a [[ ... == ... ]]
comparison. With nocasematch enabled, the lower-case indexed marker a matches
the upper-case associative marker A, so the validator emits the wrong diagnostic
instead of the indexed-array kind diagnostic. The existing string-split regression
test also asserts the later output-attribute diagnostic for an associative array;
that expectation needs to be corrected because the kind contract is the first
applicable contract and must reject the wrong array kind before checking writability.

Scope and acceptance criteria

• Make the array-kind classification case-sensitive even when nocasematch is
  enabled by the caller.
• Preserve the existing Bash 4.2-compatible implementation style and the public
  single-file library boundary.
• Add a focused regression test that exercises the validator under nocasematch
  and verifies the indexed/associative distinction; update the string-split
  regression assertion to expect the correct indexed-array diagnostic.
• Keep existing diagnostics, return codes, and caller-owned output preservation
  unchanged outside this defect.
• bats focused regression tests pass, and ./tests/validate.sh passes.

Validation

The implementation PR will record the exact focused BATS command and full
validation result.

Non-goals

• No redesign of the output-contract API.
• No compatibility shim for pre-v2 behavior.
• No unrelated refactoring or release/version changes.

Related: #521