feat: slim core feature gates

[eboody]

Summary

• gate runtime/static introspection surfaces behind the introspection feature
• gate validator batch/rebuild-report helper surfaces behind rebuild-batch / rebuild-reports
• remove the unconditional futures-util core dependency by making batch finalization sequential
• add trybuild and runtime coverage for default/no-feature and combined feature surfaces

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• bash scripts/check_readme_links.sh
• cargo test --workspace --all-features
• bash scripts/check_workspace_hygiene.sh
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --all-features --no-deps
• cargo test -p statum-macros --tests --no-default-features
• cargo test -p statum --no-default-features
• cargo check --workspace --no-default-features
• cargo test -p statum-macros --test macro_errors --features introspection
• cargo test -p statum-macros --test macro_errors --features rebuild-batch
• cargo test -p statum-macros --test macro_errors --features rebuild-reports
• cargo test -p statum-macros --test macro_errors --features 'introspection rebuild-batch'
• cargo test -p statum-macros --test macro_errors --features 'rebuild-batch rebuild-reports'
• dependency guards: no linkme / futures-util in no-default statum; linkme present with introspection; no futures-util with rebuild-batch

Audit notes

• Static added-line security scan: clean.
• Independent review: passed, no blocking security/logic/feature-plumbing issues.
• GitNexus detect-changes --scope compare --base-ref main: high risk, 32 files / 60 symbols / 7 affected processes; expected for combined fan-in scope.

Semantic authority closeout

• Claimed authority surface: feature-gated public APIs and macro-emitted introspection/rebuild helper surfaces.
• Actual observation points: source diff, Cargo feature graph checks, trybuild compile-pass/compile-fail fixtures, runtime introspection/rebuild tests, and GitNexus compare impact.
• Unsupported/open cases: no formal proof beyond these observed/tested surfaces; sequential async batch finalization is accepted unless concurrency becomes a documented contract.

Notes

• ai-slop-guard, crimee-audit, and semantic-authority-audit were required by AGENTS.md but are not available in this Hermes profile, so they were not run.
• This branch is based on local main at e57885f, which is ahead of origin/main.