Teach the codec context argument as part of the signature

[dimitri-yatsenko]

Documents the context argument added in datajoint/datajoint-python#1556, and teaches it as the signature rather than an option — so a codec written from these pages needs no change when the compatibility code is removed in 2.4 (datajoint/datajoint-python#1557).

Why now rather than at 2.4

The backward-compatible path is what makes 2.3.4 safe to ship, but it is not what anyone should be writing against. Two third-party codecs — dj-figpack-codecs#6 and dj-canvasxpress-codecs#3 — shipped the silent-wrong-store bug by copying the documented example, so the example is the thing that most needs to be right.

Changes

reference/specs/codec-api.md — a new section on context: what the four keys carry, and why threading config into _build_path() / _get_backend() is a correctness matter rather than a style one (both fall back to the global dj.config without it, which in a multi-connection process belongs to no one and resolves a different store silently instead of raising). The pre-2.3.4 form is marked deprecated with its removal release named, and a two-line migration is given.

how-to/create-custom-codec.md, explanation/custom-codecs.md, tutorials/advanced/custom-codecs.ipynb — the same signature throughout: 29 encode and 28 decode examples across the four pages, plus the _extract_context(key, context) and _codec_config(key, context) call sites that go with them.

Verification

mkdocs build --strict clean, check_links.py passes across 142 pages, check_notebook_versions.py clean.

The two notebook code cells carry no outputs, verified programmatically against HEAD, so nothing needs re-executing.

Ordering

Independent of #288 and #289 — different files, no overlap. Safe to merge in any order, though the behavior it describes only exists once datajoint/datajoint-python#1556 lands.