Add optional parallel eager loading

[elpete]

Closes #55

Issue review

Recommendation: 7/10 — implement as opt-in. Independent eager-load queries can benefit from concurrency when database latency dominates, especially for several unrelated top-level relationships. It should not be the default: extra threads add overhead, consume more database connections, and do not help a single relationship or necessarily improve low-latency local queries.

Implementation

• adds an optional parallel boolean to with, e.g. .with(["author", "comments"], true)
• runs independent top-level eager loads on separate threads
• preserves nested eager-load configuration within each top-level branch
• propagates worker failures instead of returning partial relationship data
• fails explicitly if workers do not complete within 60 seconds
• retains the existing sequential path by default and for a single top-level relationship

Test-first evidence

The public regression initially failed because both eager-load callbacks ran on the request thread. It now verifies that both relationships hydrate correctly and that each callback executes on a distinct worker thread.

Validation

• focused EagerLoadingSpec: 28 passed, 0 failed, 0 errors
• full Lucee 6 suite: 496 passed, 0 failed, 0 errors, 3 skipped
• box run-script format
• git diff --check

Uses qb@14.0.0-beta.3.