feat(esm_catalog): PR-A1b-1 — scan ingest + NetCDF

[siligam]

PR-A1b-1 — scan ingest + NetCDF (Miguel's "scan run dirs" unit)

Provenance: scan/context.py, scan/detect.py, scan/namelist.py, scan/netcdf.py, and scan/upath.py are ported from Paul Gierz's esm-tools-plus/simcat/pgierz-workbench prototype branch; scan/ingest.py is new for this decomposition.

Third slice of #1473's decomposition. Adds the scan/ingest layer that turns files into in-memory STAC (collections + items), and completes the cross-layer rewiring that the STAC foundation (#1484) deferred.

Stacked on #1484 (esm-catalog/pr-a1a-stac-foundation). Review #1483 → #1484 → this. Retargets to release as the stack merges.

What's here (+1,814 lines)

• scan/upath.py — remote-FS helpers; re-exports parse_uri/to_uri from esm_catalog.uri
• scan/namelist.py — Fortran namelist parsing (+ get_namelist_config_path, moved here from the stac layer)
• scan/netcdf.py — NetCDF scanner (xarray) → STAC metadata
• scan/detect.py — format dispatch by suffix + magic bytes (GRIB/ECHAM lazy — they land in PR-A1b-2)
• scan/context.py — collection-context resolution using the neutral CollectionContext; scan_all_namelists
• scan/ingest.py — scan_tree(root): the storage-free pipeline (list → detect/scan → resolve → STAC)
• esm-catalog scan <path> [--output FILE] CLI command
• focused unit tests (27 total in the suite, all green on 3.9 + 3.12)

Rewiring completed (deferred from #1484)

• scan/context.py uses from esm_catalog.context import CollectionContext (no local dataclass)
• CollectionContext.collection_title re-added and now honored by make_collection
• get_namelist_config_path reintroduced in the scan layer
• make_collection(ctx, namelists=, fesom_info=) called with the new pure signature; ctx.namelists_by_component populated before item-building

Scope notes

• Storage-free: scan_tree builds STAC in memory; DuckDB persistence is PR-A2.
• Cycle stays broken: scan → stac only; the A1a guard test still passes.
• GRIB/ECHAM scanners follow in PR-A1b-2.

Plan: PR-A1b plan

[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.