fix: let the limiter count only calls the circuit lets through

[costela]

Why

Wrap claimed the circuit's state for a call, including the half-open call, before any middleware ran, and the concurrency limiter took a slot before the circuit checked the call. As a result:

• calls into an open circuit waited for or took a slot instead of failing fast;
• a call the limiter rejected used up the half-open call, keeping the circuit open for another full delay;
• a call that waited for a slot entered the circuit with the state it had seen before waiting, even if the circuit had opened meanwhile.

What

• Wrap only reads the state for the middleware. The circuit decides on its current state, and claims the half-open call, once the call reaches it.
• The limiter lets calls into an open circuit through without a slot, so the circuit rejects them right away.
• The closed case of Circuit.State is split off to be inlinable, which removes the ~1.5ns the extra state read cost per call. Remaining difference vs main: within ±1.2% (benchstat, n=10, interleaved runs; command and table in the commit message).
• Tests for all three symptoms (each fails on main). The permit-leak test now uses a call that loses the half-open race, since calls into an open circuit no longer take a slot.

Behavior change for custom middleware: a half-open state passed to ObserverForCall no longer means the call gets the half-open call.

Independent of #37; the tests pass with and without it.

🤖 Generated with Claude Code

[Copilot]

🟢 Approval recommended

The implementation matches the stated behavior and includes focused regression coverage for all identified failure modes.

0 open findings

What changed in this PR

Moves circuit admission checks behind middleware delays while ensuring open-circuit calls bypass concurrency-limit slots.

Changes:

• Defers current-state and half-open admission decisions to the circuit.
• Makes concurrency limiters skip open-circuit calls.
• Adds regression coverage for slot usage and state changes while waiting.

File	Description
hoglet.go	Updates circuit state handling and middleware contract.
hoglet_test.go	Tests half-open rejection and delayed-call behavior.
limiter.go	Bypasses permits for open-circuit calls.
limiter_test.go	Tests permit release and open-circuit bypass.

🧠 Review effort: Balanced

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