docs: track encoding contract

[vycdev2]

Summary

• link the Phase K encoding progress row to design issue design: define encoding representations and failure contract #71
• add backlinks from the encoding API and stdlib-boundary sections in the design and architecture docs
• preserve the existing design text while making the unresolved representation, failure, and runtime-boundary work explicit

Verification

• cargo fmt --check — passed
• cargo test -q -p jett_driver run_pass_encoding — passed (1 matching fixture test)
• cargo build — passed
• cargo test -q — passed, including all 428 driver fixture tests
• git diff --check — passed

Risk

• Low: documentation-only backlinks; no language or runtime behavior changes.

Tracks #71

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