Add cold-start first-10 quality readout

[ohld]

Fixes FFM-1869.

What changed

• Added a repeatable true-new cold-start first-10 readout in src/stats/cold_start_quality.py.
• Added scripts/cold_start_first10_quality_readout.py for Analyst/ops execution with ANALYST_DATABASE_URL support.
• Documented the readout entrypoint in docs/analyst/README.md.
• Added focused integration coverage for true-new cohort selection, dormant/non-cold first-send exclusion, sent_at ordering, skip scoring, continuation, engine/segment splits, and top/bottom candidate meme output.

Readout coverage

The readout reports first-meme LR/continuation, first-10 LR/continuation, reached-5, reached-10, second-session, per-position quality, per-engine split, source/type/language segments, and top/bottom first-10 candidate memes.

Guardrails

• Measurement-only; no ranking, threshold, weight, queue, or experiment-assignment changes.
• Does not touch recently_liked_blender_v2 logic or readout rules.
• True-new cohort requires each user's first-ever user_meme_reaction.sent_at to be a cold-start engine send.

Verification

• ruff check --fix src/ tests/
• ruff format src/ tests/
• ruff check --fix scripts/cold_start_first10_quality_readout.py && ruff format scripts/cold_start_first10_quality_readout.py
• python3 -m compileall -q src/stats/cold_start_quality.py scripts/cold_start_first10_quality_readout.py tests/stats/test_cold_start_quality.py

Focused pytest was attempted locally with normalized DATABASE_URL, but this workspace DB is not in a clean migrated test state: Alembic hit an existing user table and the fixture then failed because meme_source was absent. CI should run this against its clean test DB.

Post-merge handoff

After merge/deploy, Analyst should run:

source .env
python scripts/cold_start_first10_quality_readout.py --lookback-days 14

Then recommend whether the next step should be a cold-start candidate/filter experiment, continued measurement, or rejecting the quality-score hypothesis.

[ohld]

STAFF ENGINEER REVIEW: CHANGES REQUESTED — two verified script-entrypoint bugs need fixing before merge.

[P2] scripts/cold_start_first10_quality_readout.py:29 - The script only uses ANALYST_DATABASE_URL when DATABASE_URL is absent. Analyst docs say this readout should use the read-only analyst DB, but a normal sourced .env can contain both variables; in that case the script keeps DATABASE_URL and may connect to the wrong DB. Verified with both vars set: DATABASE_URL stayed on the primary DSN.

[P2] scripts/cold_start_first10_quality_readout.py:64 - The documented command python scripts/cold_start_first10_quality_readout.py --lookback-days 14 fails from repo root before execution because Python puts scripts/ on sys.path and cannot import src. Verified locally: ModuleNotFoundError: No module named src.

Other checks: CI is green; ruff check, compileall, git diff --check, and /cso diff review passed. Raw SQL uses fixed templates plus bound params; no secrets found. Auto-merge was disabled before posting this change request.

[ohld]

STAFF ENGINEER REVIEW: APPROVED — Cold-start first-10 quality readout reviewed clean after the entrypoint fixes. Verified the prior blockers are fixed: ANALYST_DATABASE_URL takes priority over DATABASE_URL, and the script can import repo modules when run from repo root. Local checks: ruff check, ruff format --check, compileall, git diff --check; /cso diff review found no SQL injection or secret exposure; /codex review found no blocking correctness issues. CI lint and test are green.

[ohld]

✅ Approved + merged.