refactor(difc): extract tagSet to deduplicate concurrent map logic from Label and Capabilities

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Label and Capabilities both independently maintained a map[Tag]struct{} + sync.RWMutex with five nearly-identical methods (Add, AddAll, Remove, Contains, GetTags/GetAll). Any concurrency fix or logging change had to be applied twice.

Changes

• New internal/difc/tagset.go: unexported tagSet struct with tags map[Tag]struct{} + sync.RWMutex and shared private methods: add, addAll, remove, removeAll, contains, getAll, clear, count, isEmpty.

• capabilities.go: embed tagSet, delegate all seven public methods; debug logging preserved around the delegates; sync import removed.

• labels.go: embed tagSet, delegate Add/AddAll/Remove/RemoveAll/Contains/GetTags/IsEmpty; Union, Intersect, and Clone are unchanged — they access l.tags/l.mu directly via Go field promotion; sync import removed.

// Before — duplicated in both files
type Label struct {
    tags map[Tag]struct{}
    mu   sync.RWMutex
}
func (l *Label) Add(tag Tag) {
    l.mu.Lock(); defer l.mu.Unlock()
    l.tags[tag] = struct{}{}
}

// After — single definition in tagset.go, embedded in both types
type Label struct{ tagSet }

func (l *Label) Add(tag Tag) { l.add(tag) }

The set-algebra methods (Union, Intersect) that acquire two locks simultaneously are left untouched; field promotion keeps them working without modification.

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Pull request overview

This PR refactors DIFC tag management by extracting the shared concurrent map[Tag]struct{} + sync.RWMutex logic into a single unexported tagSet type, reducing duplication between Label and Capabilities while preserving existing behavior and debug logging.

Changes:

• Added internal/difc/tagset.go with a shared tagSet implementation covering common set operations (add, addAll, remove, removeAll, contains, getAll, clear, count, isEmpty).
• Updated internal/difc/capabilities.go to embed tagSet and delegate public methods, keeping existing debug logs around operations.
• Updated internal/difc/labels.go to embed tagSet and delegate the basic set methods, while leaving set-algebra methods (Union, Intersect, Clone) intact and still operating via promoted fields.

Show a summary per file

File	Description
internal/difc/tagset.go	Introduces shared concurrent tag-set implementation used by DIFC label/capabilities types.
internal/difc/labels.go	Embeds tagSet and delegates basic operations; set-algebra methods continue using promoted fields/locks.
internal/difc/capabilities.go	Embeds tagSet and delegates public API while preserving existing debug logging behavior.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Low