Resolve config items on demand so untaken references are never resolved

[theoephraim]

Follow-up from #1153 (and() / or()).

Problem

Resolver functions short-circuit their own nested function calls (and(false, op(...)) never calls op()), but not item references. and(false, $EXPENSIVE) and if(false, $A, $B) still resolve EXPENSIVE / A, because resolveEnvValues schedules each item only after every key in its dependencyKeys has resolved, and ref() just reads the already-resolved value.

In a full load this costs nothing, since every item resolves anyway. It matters for partial loads, where the unused dependency is otherwise never needed:

• --filter: matched items plus their transitive deps
• early-resolved @import(..., enabled=...) conditions

Proposal: resolve items on demand

Switch the graph from a push scheduler to pull-based resolution:

• ConfigItem.resolve() memoizes its in-flight promise so concurrent readers share one run (no double op() calls)
• getDepValue (only used by ref()) becomes async and awaits depItem.resolve() instead of throwing when the dep is unresolved
• resolveEnvValues becomes a Promise.all over the requested keys; deps are pulled in as they are read, still in parallel
• earlyResolve() and the transitive-dep expansion used by --filter / @import(enabled=...) go away; those paths resolve just the keys they need
• keep static dep collection for cycle detection, type generation, etc. Static cycles stay an error, so a runtime deadlock is impossible (runtime reads are a subset of static edges)

This makes if(), ifs(), remap(), and(), or() and fallback() lazy for references without marking conditional arg positions per function.

Things to handle

• "Dependency X is invalid" would only apply when X is actually read, not when it sits in an untaken branch. Arguably more correct, but error output and some tests change
• each on-demand resolve needs its own resolution context (current item, cache store) so cache hits are attributed correctly
• ordering of side effects (exec() etc.) may shift; tests relying on order (e.g. the increment test resolver) may need updates
• partial loads leave more items unresolved; audit code that assumes all deps of a resolved item are resolved (e.g. checkForSensitiveValuesInsideNonSensitiveOnes)

Related

• [Feature Request] Optional items whose resolver cannot authenticate resolve to undefined, and lazy plugin functions in untaken branches #1147 (lazy plugin functions in untaken branches) overlaps, though it also needs the unknown-function check deferred, which this does not cover

[pullfrog]

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