fix(setup): account for selected preparation and dependency failures

[LucaLin233]

Summary and scope

Project 4; base f9d5ed6, HEAD f269420, branch luno/fix-setup-module-failure-status. Two files +168/-30: linux_setup.sh and tests/test-linux-setup.sh. Retain preparation-failed candidates for selection; only selected failures affect final status. Schedule selected dependencies before consumers, block transitive consumers on failure, continue unrelated modules. Validate syntax before execution so parser exit 2 cannot become degraded success. Disabled/unselected modules and legitimate degraded behavior retain normal semantics. No workflow/package strategy/Cloudflare changes.

Validation

https://github.com/LucaLin233/Linux/actions/runs/34263421848: both required environments passed; full=false, linux-setup and setup-exec suites executed in each. Ubuntu warning/error lint passed. Tests cover discovery failures, disabled/unselected exclusion, direct/transitive blocking, dependency order/single execution, unrelated continuation, degraded status, syntax failure and real main mixed/valid exact exit status. Local bash -n, diff checks and remote/local byte comparison passed. Second-pass self-review completed, not third-party review. No production behavior tests.

Risks

No cryptographic module manifest added; existing download validation remains. Failed downloads cannot reveal enabled/platform metadata: expose failed candidates rather than guess. Existing module-specific platform skips remain unchanged; no generic platform contract introduced. Numeric display order remains but execution is dependency-first. Explicit user-confirmed external dependencies retain existing semantics. Cyclic plans retain rejection. Not a live deployment or full-suite PR validation claim.

Rollback and authorization

Recommend merge for this scope; revert eventual squash through PR if needed. Draft pending explicit merge confirmation. Project 4 not yet completed: merge and post-merge main CI are still required. No production/data changes.