fix: preserve channel-weight fallback transaction

[seonghobae]

Summary

• isolate the optional lineage_channel_weight read in an asyncpg savepoint so a missing rollout table cannot abort the enclosing rebuild transaction
• add a regression that proves the same connection remains usable after the documented fallback
• align ADR 0145 and the estimator docstring with fast-mlsirm: factor_id maps channel items to the shared trait; cluster_id carries reconstruction-group nesting

Validation

• 16 passed, 1 skipped: lineage ingestion, channel estimation, estimator script, and lineage persistence suites
• Full local suite on the stacked base: 878 passed, 18 skipped, 5 unrelated existing integration failures from local schema/provider drift; not represented as green

Follow-up to #499, which merged before these review fixes reached its head.

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[devin-ai-integration]

Devin Review found 1 potential issue.