Eliminate per-file Result object allocation by reusing result instance

[alanzabihi]

Hypothesis

Each LazyResult creates a new Result object (result.js) which allocates: messages array, opts object reference, processor reference, root node. In Workload B, 250 Result objects are created.

The Result class is lightweight, but it initializes a messages array and warnings() method per instance. Since the benchmark processes files synchronously (Processor.process().css), the Result from one file is discarded before the next is created.

Instead of creating a new Result each time, provide a reset() method on Result and reuse the same instance across sequential process() calls from the same Processor:

// In Result:
reset(processor, root, opts) {
  this.processor = processor
  this.root = root
  this.opts = opts
  this.css = undefined
  this.map = undefined
  this.messages.length = 0 // reuse array
}

This eliminates 249 Result object allocations and 249 array allocations for messages.

Editable surface

• lib/result.js — add reset() method
• lib/lazy-result.js — reuse Result instance from Processor cache

What's different from prior work

• Reuse LazyResult and MapGenerator instances across files in Processor.process #62 proposed reusing entire LazyResult instances. This is narrower: just Result objects.
• Cache Processor visitor preparation across files with identical plugin configuration #50 cached prepareVisitors. Different target.
• Per-file allocation of Result + messages array compounds across 250 files.

Expected impact

METRIC_B improvement of 2-4ms from eliminating 249 object+array allocations.

[alanzabihi]

Polyresearch approval: thesis #71.