Closes #1877.

Problem

A flow whose trigger/edge condition calls an unknown function — PRIOR(status), a typo'd isBlnk(...) — was silently accepted: build + typecheck pass, then the broken condition no-op'd the flow at runtime with no log line. The reporter hit this on @objectstack 9.5.1 while runtime-testing the objectstack-ai/templates set (signal_to_topic_promotion, cta_creation_default).

Root cause

In packages/formula/src/cel-engine.ts, compile() discarded the type-check verdict:

const checkErrors = compiled.check?.();
if (checkErrors && Array.isArray(checkErrors) && checkErrors.length > 0) { ... }

cel-js's check() returns a TypeCheckResult object ({ valid, type?, error? }), never an array — so Array.isArray(...) was always false and the verdict was dropped. An unknown function type-checks as found no matching overload for 'PRIOR(dyn)', but that result never surfaced. Every author surface (objectstack compile via validate-expressions.ts, registerFlow's validateFlowExpressions, and the agent validate_expression tool) routes through this one compile(), so all of them accepted the predicate.

Fix

Read the documented { valid, error } shape. One change closes the gap for flow conditions, validation rules, and field formulas at once.

The runtime half of the issue is already loud on current main (post-ADR-0032): the unknown-function fault message found no matching overload doesn't match the no such overload numeric-retry guard, so it propagates as an attributed flow failure rather than return false. The reporter's silent skip was on 9.5.1, which predates that work — but build validation stayed broken until this change.

Verification

• Confirmed empirically (against the real registered stdlib) that cel-js's check() cleanly separates valid predicates (record.rating >= 4, !isBlank(...), previous.status != null, every existing example flow condition) from unknown-function ones — no false positives.
• New regression tests in cel-engine.test.ts and validate.test.ts (PRIOR(), short-circuit-guarded unknown call, typo'd isBlnk, plus assertions that registered stdlib isBlank still passes).
• @objectstack/formula 95 passed · @objectstack/service-automation 187 passed · tsc --noEmit clean.

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