Counter/Lock: Redis INCR parity and safe lock release

- Counter#increment now seeds a missing key at the delta (Redis INCR/INCRBY
  semantics). Couchbase's native increment seeds at 'initial' and ignores the
  delta on creation, so the first INCR returned 0 — a drop-in replacement that
  silently undercounts by one is exactly the kind of bug nobody ever finds.
  Explicit initial: still overrides. Decrement keeps initial: 0 and its
  unsigned clamp-at-zero divergence from Redis is now documented and tested.
- Lock#release requires the CAS handle: passing nil used to delete the lock
  document unconditionally, so any caller could unlock a lock it never held.
  force_release! is the explicit administrative override.
- Lock#acquire validates a positive ttl — a nil/zero ttl would create a lock
  that survives its holder's crash forever.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

spec/redis_replacements_spec.rb

@@ -6,11 +6,16 @@ RSpec.describe 'Redis replacements' do
   describe SimplyBased::Counter do
     subject(:counter) { described_class.new }
 
-    it 'atomically increments, seeding at the initial value' do
-      expect(counter.increment('c:hits')).to eq(0)        # created at initial 0
+    it 'atomically increments with Redis INCR semantics (first INCR of a missing key returns the delta)' do
       expect(counter.increment('c:hits')).to eq(1)
-      expect(counter.increment('c:hits', 5)).to eq(6)
-      expect(counter.get('c:hits')).to eq(6)
+      expect(counter.increment('c:hits')).to eq(2)
+      expect(counter.increment('c:hits', 5)).to eq(7)
+      expect(counter.get('c:hits')).to eq(7)
+    end
+
+    it 'seeds at an explicit initial value when given' do
+      expect(counter.increment('c:seeded', 1, initial: 100)).to eq(100)
+      expect(counter.increment('c:seeded')).to eq(101)
     end
 
     it 'decrements and resets' do
@@ -19,6 +24,11 @@ RSpec.describe 'Redis replacements' do
       expect(counter.reset('c:n', 0)).to eq(0)
       expect(counter.get('c:n')).to eq(0)
     end
+
+    it 'clamps decrement at zero (unsigned Couchbase counters; Redis would go negative)' do
+      counter.reset('c:clamp', 1)
+      expect(counter.decrement('c:clamp', 5)).to eq(0)
+    end
   end
 
   describe SimplyBased::Cache do
@@ -45,6 +55,19 @@ RSpec.describe 'Redis replacements' do
       expect(lock.acquire('job:1', ttl: 30)).not_to be_nil # released → free
     end
 
+    it 'refuses to release without the CAS handle' do
+      lock.acquire('job:guard', ttl: 30)
+      expect { lock.release('job:guard', nil) }.to raise_error(ArgumentError)
+      expect(lock.acquire('job:guard', ttl: 30)).to be_nil # still held
+      lock.force_release!('job:guard')
+      expect(lock.acquire('job:guard', ttl: 30)).not_to be_nil
+    end
+
+    it 'rejects a missing/zero ttl (a crashed holder must never leave an immortal lock)' do
+      expect { lock.acquire('job:immortal', ttl: nil) }.to raise_error(ArgumentError)
+      expect { lock.acquire('job:immortal', ttl: 0) }.to raise_error(ArgumentError)
+    end
+
     it 'runs a critical section with with_lock' do
       ran = false
       lock.with_lock('job:2') { ran = true }