fix(embed): gate embed backlog on true unembedded count, not IVF index lag

[tenequm]

Problem

After a fresh pond copy over S3, the embed stage shows a progress bar that looks permanently hung on what is actually a no-op.

Root cause: pond copy carries already-embedded rows into the destination with the vector intact (projection: None), but the IVF_SQ vector index has not folded those rows yet. The embed stage gated its backlog on unindexed_vector_backlog - the index-lag count - so it read ~104,994 "unindexed" rows while 0 were actually unembedded. The stage then spins up a progress bar sized to ~105k that can never make progress, because there is nothing to embed.

The index-lag count is a poor proxy for the embed backlog: it counts "rows the index hasn't absorbed," which after a copy is a large set of fully-embedded rows.

Approach

Gate on embed_backlog_count instead - the true eligible-unembedded count (search_text present, embedding_model null), read off the narrow embedding_model column. On the post-copy store this correctly reports 0, so the stage stays silent (the summary's indexes line confirms "semantic ready" downstream).

The narrow-column scan was the expensive thing that originally motivated the index-lag proxy, but the v0.10.1/v0.10.2 backlog work already made embed_backlog_count cheap (manifest-only / narrow-column read), so there is no longer any perf reason to gate on the coarser proxy.

swapped (model-swap detection) is unchanged - it still drives the force/bail path, the IVF drop, and the bar_total == 0 && !swapped silent-skip.

Changes

• src/main.rs: run_embed_stage_with_limit reads store.embed_backlog_count() unconditionally; replaces the swapped ? embed_backlog_count : unindexed_vector_backlog branch.
• src/sessions.rs: removes the now-unused Store::unindexed_vector_backlog (no other callers).

Net: +9 / -23.

Validation

• cargo fmt --check clean
• cargo clippy --locked -- -D warnings clean
• cargo test --locked - 235 passed, 0 failed (169 lib + 24 bin + 42 integration), incl. embed_backlog_count_tracks_eligible_unembedded_rows and embed_worker_drains_the_backlog