fix(search): preserve coordinator filter and degradation contract

[ohdearquant]

Summary

• validate the complete KG search request once before coordinator fan-out, including granular/legacy kind reconciliation and substrate-specific filters
• forward every applicable filter to each backend and reject malformed, contradictory, or wrong-substrate inputs instead of silently broadening searches
• preserve typed partial-result metadata (partial and missing_backends) through MCP and kkernel single, batch, chain, presentation, audit, and frame-budget paths
• make backend task panics explicit per-backend failures and cover the cross-surface contract with regression tests and operator documentation

Why these issues belong together

Both failures occur at the same coordinator interception boundary: one loses accepted search filters before fan-out, while the other loses degradation metadata after fan-out. Keeping the fixes together lets the boundary use one validated request type and one typed intercepted-dispatch result without duplicating compatibility glue across MCP and kkernel.

Verification

All checks passed at exact commit fb1feb7e92ea60e36a811a834ed22f381b3100fa:

• cargo test --manifest-path crates/Cargo.toml --workspace
• cargo check --manifest-path crates/Cargo.toml --workspace
• cargo clippy --manifest-path crates/Cargo.toml --workspace --all-targets -- -D warnings
• cargo fmt --manifest-path crates/Cargo.toml --all -- --check
• RUSTDOCFLAGS=-Dwarnings cargo doc --manifest-path crates/Cargo.toml --workspace --no-deps

An independent review found no actionable issues.

Closes #1370
Closes #1377

AI-assisted contribution: Codex prepared this change and PR description.