feat(composer): add flow component debug package with OTEL-style global provider

[Copilot]

Introduces internal/composer/debug — an OTEL-inspired global provider for flow component debugging. Designed to be called on every FlowComponent invocation with the noop path costing zero heap allocations.

Design

• Debugger interface — StartAction(ctx, componentName) Action begins a debug session per component call.
• Action interface — End(Outcome) records the result; Outcome is a plain value struct (StatusCode, Duration) with no pointers.
• GetDebugger() / SetDebugger() — global accessors backed by atomic.Pointer[Debugger] for lock-free reads on the hot path.
• Noop path — default provider returns pre-allocated singleton *noopAction; assigning a pointer to an interface is two-word stack storage, no heap escape. Verified at 0 B/op, 0 allocs/op.

Intended usage in a FlowComponent

action := debug.GetDebugger().StartAction(ctx, "my-component")
defer action.End(debug.Outcome{StatusCode: wrapper.StatusCode(), Duration: elapsed})

No init() functions; global state bootstrapped via var global = newGlobal() to satisfy the gochecknoinits linter.