A new `template` value source evaluates a hardened, sandboxed Jinja
expression over the live values of other value sources — the system's
first float combinator.
Backend:
- Shared engine (utils/template_expr.py): ImmutableSandboxedEnvironment with
filters/tests and auto-injected globals stripped; only min/max/abs/round/
clamp exposed; rejects **, string/collection-literal repetition, attribute
access and non-global calls; NaN/inf-safe result coercion.
- TemplateValueSource model + TemplateValueStream runtime: compile-once,
primitives-only eval context, raw[name] exposure, eval_interval throttle,
ref-counted input acquire/release, rename-safe hot-update.
- Validation: unbound-variable + reserved-name rejection, reference
cycle/depth guards (depth-only at create, full cycle at update), runtime
acquire() depth backstop, and delete referential-integrity.
- API: Create/Update/Response schemas + discriminated unions, _RESPONSE_MAP,
and an advisory POST /value-sources/validate-template endpoint.
- Demo seed: a static source plus a template combinator example.
Frontend:
- Editor modal section: repeatable inputs list (EntitySelect rows), a
zero-dependency Jinja syntax highlighter, a hints/reference panel, and a
debounced live validator that gates Save (stale-response-safe).
- Graph editor: read-only template node with one edge per input.
- i18n (en/ru/zh), icon, and card rendering.
Tests: engine, stream, factory/cycle, validate endpoint, and demo seed.
