Catalog sdmx-core custom overrides

# Problem

`services/sdmxsource/` contains ~21 custom classes that override or replace
sdmx-core implementations to work around upstream bugs, missing extension
points, or spec violations. The reasons are scattered across class comments,
commit messages, and prior design docs; there is no single reference that
maps each override to the upstream class, the underlying bug, and whether
the workaround is still needed after the 2.4.0 upgrade.

## Goal

Produce a reference catalog: per override, document target upstream class,
mechanism, wiring site, behavioural delta, the bug it works around,
severity / blast radius, status against sdmx-core 2.4.0, and risks of
removal. Identify candidates for upstream contribution, candidates for
deletion, and the most fragile / load-bearing patches.

## Design

See [`docs/designs/022-sdmxsource-overrides-catalog/DESIGN.md`](../designs/022-sdmxsource-overrides-catalog/DESIGN.md).