feat(Builder): add withSpecification(), a contribution seam before resolution

[DerManoMann]

Overview

Metadata that describes an API but has no reflector cannot reach the spec pipeline.
A translator turns one attribute into another and needs something to read it off; an
entity registry or a serializer's configuration carries no attribute anywhere. The
only way in today is an augmenter, which runs after resolution, so a $ref inside
what it adds stays unresolved.

Builder::withSpecification() hands the assembled Specification to a callable
between assembly and resolution. What it adds is resolved, augmented and compiled
with the scanned sources. The pipeline reads a reflector, not a source file, so a
contribution that carries one gets the scanned treatment, docblocks and parameter
types included, and one without gets an operation id from method and path since
there is no declaring code to name.

With a second deferred hook on Builder, the docblocks' silence on when each hook
runs and whether repeated calls accumulate became ambiguous, so each one now says.

Changes

• Add Builder::withSpecification(), with SpecificationContributionTest
• Derive an operation id from method and path in OperationIds for an operation
  with no reflector
• State on every Builder hook when it runs and whether repeated calls accumulate
  or replace
• Add "Contributing to the Specification" to the extension-points guide, with an
  executed snippet and its expected output
• Add the hook to the builder reference and the modes table
• Drop the builder reference's closing example, which repeated the page's other
  snippets