Clean deprecated code and repository residue

[a19q3]

What changed

• split the monolithic backend into the documented ABI, assembler, call, Cell-operation, collection, expression, frame, runtime, and schema modules
• remove the unreachable external RISC-V toolchain fallback and deprecated fail-closed CKB adapter methods
• replace deprecated serde_yaml with serde_yaml_ng
• remove crate-wide lint exemptions and clean up dead test-fixture state
• delete tracked Playwright traces and unused design captures, and prevent their return through source policy
• correct the E2400 diagnostic contract and advance the editor/website submodule pins

Why

The repository contained a large backend orchestration file, dead compatibility implementation paths, an explicitly deprecated Rust dependency, stale diagnostics, obsolete lint exemptions, and generated UI artifacts tracked as source.

Impact

The compiler behavior remains covered by the same backend/runtime tests, while module ownership, diagnostics, dependency health, and repository hygiene are materially clearer.

Validation

• ./scripts/cellscript_gate.sh ci
• 751 compiler unit tests
• 218 iCKB differential tests
• workspace Clippy with -D warnings
• Registry Type Script reproducible build and CKB-VM tests
• production/Pudge Testnet website parity
• git diff --check and native source policy