feat: introduce v2 module with clean API

[basgys]

What changed and why

v1 accumulated several mutable package-level globals (IsRejectedError, ClientSideRejectionError, Now) that made it impossible to configure two throttle instances differently and caused subtle races in tests. This PR introduces github.com/deixis/bulwark/v2 as a clean break: all configuration lives on the instance, there are no globals left to mutate, and the API surface is trimmed to remove the pieces that didn't pull their weight.

Breaking changes from v1

v1	v2
bulwark.IsRejectedError = func(...) {...}	WithRejectedErrorFunc(fn) option on each instance
bulwark.ClientSideRejectionError (var)	bulwark.ErrClientSideRejection (sentinel, compare with errors.Is)
bulwark.Now = func() time.Time {...}	WithNow(fn) option on each instance
OnInvalidPriorityClamp / OnInvalidPriorityError / OnInvalidPriorityPanic (vars)	ClampInvalidPriority / RejectInvalidPriority / AssertValidPriority (plain functions)
WithAdaptiveThrottle[T](...)	Throttle[T](ctx, at, priority, fn) — single generic entry point
WithAcceptedErrors(fn)	Removed — superseded by WithRejectedErrorFunc

Everything else (WithRejectedErrorFunc, WithClientSideRejectionError, WithNow, WithRandomSource, WithPriorityValidator, priority constants, RejectedError, fallback signatures) carries over from v1 unchanged.

Files

• v2/adaptive.go — core throttle, defaults set at construction time, no global fallbacks
• v2/validator.go — validators as plain functions instead of mutable vars
• v2/priority.go, v2/context.go, v2/counter.go — carried over from v1, unchanged
• v2/adaptive_test.go, v2/context_test.go — full test suite targeting the v2 import path
• README.md — updated to v2 import path, errors.Is checks, and new classifier API

---

Note

Medium Risk
Introduces a new v2 Go module with a largely new public API surface and behavior around error classification/configuration, plus a dependency upgrade; while v1 remains, consumers migrating to v2 will need careful review and testing.

Overview
Adds a new github.com/deixis/bulwark/v2 module (Go 1.26) that removes mutable package-level configuration by making rejection classification, client-side rejection error, time source, RNG, and priority validation per-throttle options, and provides a single generic Throttle[T] entry point alongside AdaptiveThrottle.Throttle.

Updates docs to the v2 import path and new APIs (errors.Is(err, bulwark.ErrClientSideRejection), WithRejectedErrorFunc, DefaultRejectedErrorFunc), adds a comprehensive v2 test suite, upgrades github.com/deixis/faults to v1.0.1, and updates GitHub Actions to run tests for both modules with different Go versions.

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