feat(level-migrate): give semi_abstract an exit, and stop demoting pinned rows

[MXAntian]

The sink

semi_abstract was terminal. Rows arrived from meta_knowledge and never left, so an entry that stopped being useful kept competing at recall weight 1.0 with live ones, forever.

Measured on a 10k-row store:

1,276 rows  past 30 days, zero recalls, sitting in semi_abstract
   78 rows  in concrete_trace        ← nothing demotes in, only promotes out

Three changes

1. Second stage — semi_abstract → concrete_trace at ≤2 recalls && >90 days.

Deliberately stricter than the stage above it (>30d && 0). Reusing that arm would cascade a row meta → semi → concrete inside a single night — that is not ageing, it is deleting by ladder. Surviving 90 days on two recalls is the evidence this asks for.

Dry run on the live store:

meta → semi        37   ← existing arm, the daily trickle
semi → concrete   475   ← the backlog, drained once
after:  meta 5276 · semi 2849 · concrete 553 (was 78)

2. is_anchor / is_pinned are exempt from demotion.

Both flags mean "keep this surfaced", set by hand against a quota. The nightly job was overruling them silently — the flag stays set while the weight it was meant to guarantee drops. Demotion only; promotion still applies, since raising a pinned row never contradicts the pin.

Pre-existing on the meta arm. Adding a second demotion without fixing it would have doubled the exposure.

3. Direction judged by level rank. The tally read old_level === 'meta_knowledge' ? demoted : promoted — correct only while meta was the sole source of demotions. semi → concrete would have counted as a promotion, and the nightly report would have shown the ladder running backwards.

Test

New stage · both exemptions · the one-step-per-run brake · a 40d semi untouched (the 30d arm stays meta-only) · continued descent on a later run · direction counting.

306 passed / 0 failed. Added to CI.

🤖 Generated with Claude Code