fix(topic): bind artifact ids to fitted draw basis

[seonghobae]

Summary

• consume stable topic identities from the joint posterior draw set after precision construction
• derive emitted topic count and identity order from that fitted, digest-bound basis
• cover the artifact identity order in the execution contract

Why

PR #284 accepted caller memory beside fitted coordinates. This repair makes the joint-precision/draw producer the single downstream authority for coordinate identity order, so artifact serialization cannot drift from the basis included in posterior_draw_set_id.

Verification

• rustup run 1.98.0 cargo fmt --all -- --check
• rustup run 1.98.0 cargo test -p analysis_engine topic_context_posterior --all-features (11 passed)
• rustup run 1.98.0 cargo test -p analysis_engine --test topic_lineage_execution_contract --all-features (3 passed)

Stacked on #284.

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