fix(validate): honor effective env cascade

[gfargo]

Summary

Advance #248 by making strut <stack> validate evaluate required_vars against the same effective environment cascade used by deployment.

Changes

• replace the required-vars helper's base-file-only reload with validate_env_file
• include common.env, the selected stack/project base env, the active topology host layer, and generated stack/host layers in normal last-wins precedence
• preserve the existing missing-env warning, helper signature, diagnostics, counters, and command exit behavior
• retain the previous tolerance for loader failures under set -e

Validation

• bats tests/test_validate.bats tests/test_utils.bats tests/test_env_gen_layer.bats tests/test_topology.bats tests/test_cli_resolve.bats </dev/null — 178 passed
• bats tests/ </dev/null — 2,650 passed, 2 expected skips
• shellcheck -x -S warning lib/cmd_validate.sh
• bash -n lib/cmd_validate.sh
• git diff --check
• semantic review approved with no actionable findings

Advances #248.