mempool: implement StandardPolicyEnforcer for RBF and limits

In this commit, we introduce the StandardPolicyEnforcer, a new component
that separates policy decisions from graph data structure operations. This
clean separation enables easier testing, different policy configurations,
and clearer code boundaries between what the mempool stores versus what
policies it enforces.

The PolicyEnforcer interface defines five core validation methods that
work against a minimal PolicyGraph interface. This abstraction allows
policy enforcement to operate on any graph-like structure without tight
coupling to the specific txgraph.Graph implementation.

The StandardPolicyEnforcer implements Bitcoin Core-compatible policies:

RBF (Replace-By-Fee) Support:
We implement full BIP 125 RBF validation including both explicit signaling
(sequence numbers ≤ 0xfffffffd) and inherited signaling where transactions
inherit replaceability from unconfirmed ancestors. The recursive ancestor
traversal uses a cache to avoid redundant graph walks when checking deep
transaction chains.

The ValidateReplacement method enforces all five BIP 125 rules: eviction
limits, no parent spending, higher fee rates, sufficient absolute fees,
and no new unconfirmed inputs. This matches Bitcoin Core's logic and
ensures compatibility with the existing network.

Ancestor/Descendant Limits:
Bitcoin Core limits transaction chains to 25 ancestors and 25 descendants,
each with a maximum total size of 101 KB, to prevent unbounded chain
growth in the mempool. We implement identical limits with clear error
messages that specify which limit was exceeded and by how much.

Fee Rate Validation:
The ValidateRelayFee method implements minimum relay fee checking with
an exponentially decaying rate limiter for low-fee transactions. This
prevents spam while allowing some free transactions through, using the
same 10-minute half-life decay as Bitcoin Core.

The PolicyConfig structure provides sensible defaults matching Bitcoin
Core but allows operators to customize limits for different network
conditions or use cases. All policy violations return specific error
types that enable callers to distinguish between different rejection
reasons.

Author: Roasbeef