R2: the Null Object is named and no argument is nil

[buzzdan]

Summary

Follow-up to #28, from the first re-run of the case-e evals on 2.10.1.

The 2.10.1 R2 example defaulted a nil sink inside the constructor:

if sink == nil { sink = DiscardSink() }      // or make callers pass it

That keeps NewReporter(nil) legal and only relocates the nil-check. The re-run of case-e-nils-refactor (two passes) showed both halves of the gap:

• Pass 1 reproduced that exact shape: io.Discard composed inline in the constructor, no named DiscardSink(), a test still calling NewReporter(nil, ...).
• Pass 2 reached the option-based constructor with no nil anywhere, then introduced a one-implementation Sink interface with a noopSink{} for the no-op, which R11 and R2 already forbid but the refactoring skill's move table only names the move and points at the rule.

The review side did move: fix-null-object went 0/2 → 2/2 and one run wrote func DiscardSink() *Sink { return NewSink(io.Discard) } verbatim.

Changes (regenerated into the plugin):

• core/rules/R2-self-validating-types.md: the design point says no parameter accepts nil to mean "default", names the constructor guard as the smell in a new place, and the example supplies DiscardSink() and time.Now through an option: NewReporter(opts ...Option), NewReporter(WithSink(sink)) in production, NewReporter(WithClock(fixed)) in tests. The Introduce Null Object fix move says the default is named and passed or supplied by an option, never inferred from nil.
• core/skills/refactoring/SKILL.md: the move table lists Introduce Null Object under R2 for optional collaborators and carries the Go shape beneath it: a named value of the existing concrete type (DiscardSink() composing io.Discard) supplied through an option or passed by name; never a new interface for the no-op (R6), never a nil parameter meaning default (R2).
• Changelog entry under Unreleased. task check, lint-core (Residue table refreshed) and the docs gate pass.

Companion grader change: buzzdan/ldd-evals#3.

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