Embedding-based cross-document entity clustering (follow-up to #431)

[CheeryProgrammer]

Follow-up to #431, which shipped exact normalized-string cross-document entity resolution for the batch path (merge proposed nodes sharing a normalized canonical name or alias after a batch applies).

This issue is the semantic half: catch same-entity mentions that the normalizer cannot — near-synonyms and typos ("Postgres" vs "PostgreSQL", "k8s" vs "Kubernetes", "OrderSvc" vs "OrderService") — using embedding similarity, not string identity.

Why it's separate

• Fresh extraction proposals carry no summary embedding. resolveOrProposeNode inserts a node with just a canonical name + aliases; summary_embedding is NULL until a summary is written. store.ProposeSemanticMergeCandidates (the existing review-time semantic pass) operates on summary_embedding, so it finds nothing among fresh proposals. Semantic batch clustering first needs an embedding for each new proposal (embed the canonical name/alias text, or a synthesized summary).
• Threshold tuning + false-merge risk. Unlike exact identity, embedding distance is fuzzy; an over-eager threshold merges genuinely distinct entities. Needs a conservative threshold (tighter than SemanticDupThreshold = 0.15) and likely a secondary guard (type agreement, alias token overlap) before auto-merging even a proposed node.

Scope

• Give batch-created proposed nodes an identity embedding (name/alias text).
• After resolveBatchDuplicates' exact pass, cluster the remaining proposals by embedding distance under a conservative threshold (reuse ProposeSemanticMergeCandidates/SemanticDupThreshold machinery), merge only high-confidence pairs, leave the rest as separate proposals for Consolidate review.
• DB-test with fixture embeddings (inject vectors directly, like existing vec() test helpers) so it's deterministic.

Constraints

• Same conservatism as Cross-document entity resolution for batch extraction (follow-up to #420) #431: proposed nodes only, scope-keyed, a human still reviews the merged proposal.
• Logic in core; reuse consolidate*.go primitives.