fix: validate caller-owned output attributes

[codeforester]

Closes #509

Summary

• Define scalar, integer, indexed-array, and associative-array output contracts across public APIs.
• Reject readonly/nameref outputs and attributes that would coerce values before publication.
• Document the output contracts and add cross-module regression coverage, including shell-option and nameref edge cases.

Validation

• All eight affected BATS module suites pass individually.
• tests/bash-option-contract.sh passes for every configured shell-option combination.
• tests/docs-contract.sh, tests/lint-warnings.sh, ShellCheck, shfmt on modified Bash files, and git diff --check pass.
• Full pinned-Docker shfmt contract could not run locally because the Docker daemon is unavailable; hosted CI will run it.

[codeforester]

Confirmed fixed. base_app_config_get now dynamically resolves integer vs scalar output kind from the config's declared type — verified a declare -i destination for an integer-typed config value now populates correctly instead of erroring. The array-kind check no longer uses a plain substring match; verified under shopt -s nocasematch that both directions (associative array claimed as indexed, and vice versa) are now correctly rejected while genuine matches still pass.

[codeforester]

The failing nocasematch validation is resolved in dependent PR #525.

That PR is based on this branch and adds the missing case-sensitive array-kind
classification plus regression coverage. Its complete hosted validation is
green, including both Validate (ubuntu-24.04) and Validate (macos-14), as
well as quality, release, compatibility, downstream, and branch-policy gates.

Merge #525 into this branch before merging #521.