docs: clarify contract boundary

CONTEXT.md

@@ -39,9 +39,21 @@ _Avoid_: reputation, rating, rank
 ### Payments & fraud
 
 **Stake**:
-Tokens a node locks as collateral before it can serve paid inference. Slashed on fraud.
+Tokens a node locks as collateral that can be slashed for fraud. Stake protects the network economically, but route selection is not based on a node's token balance.
 _Avoid_: deposit, bond, escrow
 
+**Tax**:
+The share of caller payments distributed to compute nodes as rewards. Taxes are weighted by completed work and historical node speed so faster, larger nodes earn proportionally more.
+_Avoid_: fee, toll, commission
+
+**Caller Credit**:
+Free starting balance granted to a new caller/API key so they can try the network before topping up.
+_Avoid_: signup bonus, faucet, airdrop
+
+**Free Compute Job**:
+Work a compute node performs without earning immediate rewards, usually during probation or bootstrap phases.
+_Avoid_: unpaid labor, warmup request
+
 **Slash**:
 The act of reducing a node's stake as a penalty for a proven fraud incident.
 _Avoid_: penalize, burn, fine
@@ -62,6 +74,10 @@ _Avoid_: trial period, warmup, grace period
 Our native Solana L2 token. Used by nodes for staking and received as inference rewards. Clients never need to hold it.
 _Avoid_: coin, reward token, native token
 
+**Contract Boundary**:
+The Python interface in `packages/contracts` that represents registry, payment, and settlement behavior. During the prototype it is implemented by deterministic local wrappers; later the same boundary is backed by real Solana programs.
+_Avoid_: mock contract, fake chain, temporary hack
+
 **Validator**:
 A trusted node (or the tracker itself) that re-runs a sample of inference requests to detect fraud.
 _Avoid_: auditor, checker, referee