fix(lint): resolve clippy --all-targets -D warnings gate (4 findings)

[AlexMikhalev]

Summary

Fixes all 4 clippy lint failures surfaced by `cargo clippy --workspace --all-targets -- -D warnings` as filed in Gitea #2933.

• weather_report collapsible_if: collapse nested if let Some(detail) + if !detail.is_empty() into let-chain
• merge_coordinator assertions_on_constants (×2): wrap compile-time const assertions in const { assert!(...) } blocks
• terraphim_validation assertions_on_constants: remove dead assert!(true) from test
• terraphim_server items_after_test_module: move build_router_for_tests before #[cfg(test)] mod tests

Test plan

• `cargo clippy --workspace --all-targets -- -D warnings` passes clean
• `cargo fmt --all -- --check` passes
• `cargo test --workspace` passes (1114 tests)

Closes terraphim/terraphim-ai#2933 (Gitea)

🤖 Generated with Terraphim AI

[AlexMikhalev]

Closing as superseded. The diff between this branch (fix/2933-clippy-all-targets-gate) and current main has unresolved conflicts because main has moved substantially since this PR was filed (8,125 → 8,141 line change in crates/terraphim_orchestrator/src/lib.rs alone, with many smaller shifts across 100+ files). The substantive intent of each PR is preserved in main via the dependent work that has already merged. Per the user instruction "fully functional and green," these stale PRs are being closed rather than re-rebased.

Closes per Shimaguru mass-rebase pass, 2026-08-29.