What is the difference between value receivers and pointer receivers in Go methods?

type Counter struct{ count int }

// Value receiver — operates on a COPY of Counter
func (c Counter) Value() int { return c.count }

// Pointer receiver — operates on the ORIGINAL Counter
func (c *Counter) Increment() { c.count++ }
func (c *Counter) Reset()     { c.count = 0 }

c := Counter{}
c.Increment()           // Go auto-takes &c c.count becomes 1
fmt.Println(c.Value())  // 1

// Method set rules — critical for interface satisfaction:
// Value T:   method set = {value receiver methods}
// Pointer *T: method set = {value receiver methods} ∪ {pointer receiver methods}

type Stringer interface{ String() string }

type MyType struct{ v int }
func (m *MyType) String() string { return fmt.Sprintf("%d", m.v) } // pointer receiver

var s Stringer = &MyType{42}  // OK   — *MyType has String()
// var s2 Stringer = MyType{42} // COMPILE ERROR — MyType (value) does NOT have String()

// Rule of thumb for choosing receiver type:
// Use POINTER receiver when:
//   1. The method must mutate the receiver
//   2. The receiver is large (avoid expensive copies)
//   3. The struct has non-copyable fields (sync.Mutex)
// Use VALUE receiver when:
//   1. The method is purely read-only
//   2. The type is small and immutable (time.Time, net/netip.Addr)
// Consistency rule: if ANY method uses pointer receiver, use pointer for all