feat(esm_catalog): PR-A3 — shard federation (merge_shards)

[siligam]

Summary

• Adds storage/federation.py: merge_shards(shard_paths, output_path) — uses DuckDB ATTACH to copy collections, items, and collection_item_props rows from any number of per-experiment .duckdb shards into a single global catalog file. Rows are upserted (INSERT OR REPLACE / INSERT OR IGNORE) so the command is safe to re-run as shards are refreshed. Missing paths are skipped with a warning.
• Adds esm-catalog merge <shard>... --output <global.duckdb> CLI subcommand.
• 7 new tests: two-shard merge, idempotency (no duplicates on re-merge), incremental merge into a non-empty global, missing-shard tolerance, property index propagation, and CLI end-to-end.

This is Miguel's third unit — "sync shard to global DB". Together with A1b-1/A1b-2 (scan) and A2 (shard), the full ingest pipeline is now:

esm-catalog scan <exp-dir> --db exp-alpha/catalog.duckdb
esm-catalog scan <exp-dir> --db exp-beta/catalog.duckdb
esm-catalog merge exp-alpha/catalog.duckdb exp-beta/catalog.duckdb --output global.duckdb

Test plan

• pytest src/esm_catalog/tests/test_federation.py -v — 7 tests pass locally
• Full suite pytest src/esm_catalog/tests — 62 tests, 0 failures
• CI green on Python 3.9 + 3.12

🤖 Generated with Claude Code

[siligam]

Rebased onto the corrected PR-0 base (#1483) — test relocation to tests/test_esm_catalog/, restored dedicated CI workflow at the new path, and a couple of CI fixes (doctest-collection exclusion, eval_type_backport for pydantic on Python 3.9). No change to this PR's own diff/scope. CI is green.