Schema: each expression field says which values and variables it accepts

The registry knows, for every expression field, the constants it may
evaluate to and the variables its conditions may read, and enforces both.
The schema served to INBUXA Admin described every one as a bare
x:Expression, so the console could offer nothing better than free text.

tools/fork/expr-schema.py reads those contexts from the generated registry
code and writes them onto each field's type as
expression: {constants, variables}. All 124 expression fields are covered.
CI runs it with --check so the schema can't drift from the registry.

tools/fork/README.md

@@ -112,3 +112,18 @@ a fresh copy for each one. See `docs/spec/compat-tests.md`.
 tools/fork/run-compat.sh --store /srv/inbuxa-copy/rocks.db \
     --admin '[email protected]:PASSWORD' --recordings ~/compat
 ```
+
+## expr-schema.py
+
+Writes each expression field's allowed constants and variables, read from the
+generated registry code, into the schema the server serves INBUXA Admin
+(`resources/schema/schema.json.gz` and its checksum). The admin uses them to
+offer plain choices instead of a free-text box.
+
+```bash
+tools/fork/expr-schema.py          # update the schema
+tools/fork/expr-schema.py --check  # exit 1 if it's out of date (CI)
+```
+
+Re-run it after anything that regenerates the registry, an upstream import
+included.