[joepetjr]

What

seo-manager report now handles bad inputs and an empty join cleanly instead of crashing or printing an empty report with no explanation.

• A missing or unreadable ledger or videos.yaml, a ledger line that is not valid JSON (or has no timestamp), unparseable YAML, a videos.yaml without a top-level videos list, an entry missing id/title/date, and a bad date each print one line naming the cause to stderr and exit 2. No traceback.
• A join that matches nothing still exits 0, but the report and stderr now say why: no videos, no events, no video id in common, or no views measurement.
• An unwritable --out path exits 2 with one line.
• README documents the behaviour.

Why

Before this, a bad input gave a raw JSONDecodeError/KeyError/FileNotFoundError traceback, and an empty join produced a table of zeros with no hint whether the cause was the wrong file, a stale ledger, or a missing metric.

Evidence

Red on base: the new tests/unit/test_perf_report.py copied onto origin/main gives 11 failed, 11 passed (all 11 new CLI error/empty-join tests fail, for example FileNotFoundError traceback on an unwritable --out).

Green on this branch, full suite (python -m pytest, run through the slot gate):

Required test coverage of 66.0% reached. Total coverage: 88.73%
================= 1495 passed, 20 xfailed, 2 xpassed in 36.80s =================

0 failed. Lint with the CI-pinned ruff 0.15.22 (ci/tools-requirements.txt): ruff check . all checks passed; ruff format --check . 172 files already formatted.

Diff: 4 files, 272 insertions, 28 deletions (300 total).

UI evidence: terminal-only change, no screenshots applicable.