Repair embedded-runtime CI contract findings

[MrScripty]

Repairs the embedded-runtime area exposed by the strict aggregate Clippy gate, preserving the earlier repair stack through PR41.

Separately reviewed milestones:

• Mechanical defaults, borrow and match cleanup, with the full default-feature embedded unit suite added to required focused CI.
• Private transparent artifact-write failure payload indirection, preserving exact Display and the concrete WorkflowServiceError source/downcast contract without changing sink Result signatures.
• Private package-facts and snapshot indirection, preserving raw DTOs, source selection and clone isolation.

Validation: pinned full cargo fmt passes locally and at the preceding hosted head. At b73c5d1, actual embedded execution passed 449 tests including the new concrete source-chain tests, with three failures retained: one deferred-reservation lifecycle fixture and two production-facts validation/publication fixtures. Clippy improved from 13 to 6 embedded findings. Current head 788ce31 has fresh Quality, Headless and Runtime Separation runs pending; no local Rust build is claimed.

The full embedded suite remains required. Remaining private/public arity work and fixture corrections will be separately reviewed commits here. Production dependency audit and frontend critical-lint fixes remain on PR21/PR22 for final integration. No merge qualification or complete external review is claimed.

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