chore(index): name the FTS snippet column + record the declined #193 result

[CryptoJones]

Part of the v7.0.0 batch. Closes out #193 as not planned — the feature is not in this PR, only its fallout.

What happened

Built the #193 synthetic contextual prefix behind OMI_CONTEXTUAL_CHUNKS, evaluated it both ways on the live 784-note vault with the semantic leg on:

	OFF	ON
recall@1	60.00%	60.00%
recall@5	60.00%	60.00%
MRR	0.640	0.640
index size	13,908 KiB	18,248 KiB (+31%)

Three of five labelled cases were already rank 1 both ways; the two misses moved 7→8 and 13→11. Neither crossed k=5.

The premise didn't hold for omind. The issue assumed a mid-note chunk "competes on its own words alone" — true of claude-obsidian, not of us. _ingest has always embedded title + heading + tags + chunk.text, and chunks_fts has always given BM25 separate title/heading/tags columns. omind has been doing contextual retrieval without the name; the prefix's only real addition is the Summary, which on descriptive titles restates an already-indexed signal.

Full data, including the n=5 caveat, is in the issue and in BACKLOG.md under Not planned.

What this PR actually contains

The experiment exposed a genuine trap. FTS5's snippet() takes a column number:

snippet(chunks_fts, 3, ...)   # `text` — until someone inserts a column before it

Adding context ahead of text silently re-pointed every excerpt at the wrong column — no error, just excerpts reading "Prefs — accessibility. Section: Details." instead of the matched text. Three existing tests caught it, which is the system working.

The literal is now _FTS_TEXT_COLUMN with a comment recording why it must move in step with the schema. Small, but it converts a silent failure into an obvious one.

Gates

ruff check . · mypy src (strict) · pytest (869 passed) · pip-audit — green locally. No behavior change.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between d6b5497 and 8bad2bf.

📒 Files selected for processing (3)

• BACKLOG.md
• CHANGELOG.md
• src/omind/searchindex.py

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