feat(core): introduce side-effect-free core entrypoint

[gustavovalverde]

Problem

Some consumers need lightweight utility APIs (for example KmsEIP712) without triggering WASM/native bootstrap at module import time.

Today, those APIs are effectively reachable through ./node / ./web entrypoints, which pull initialization paths and transitive WASM/native dependencies. In browser-targeted bundlers, this can cause resolution failures (fs/native paths) or unstable build behavior.

Why This Is Needed

This is currently impacting downstream tooling such as fhevm-mocks:

• fhevm-mocks needs relayer-sdk EIP-712 helpers for mock flows.
• Importing them through @zama-fhe/relayer-sdk/node couples the import to heavy runtime dependencies.
• As a workaround, downstream code could vendor copies of helper logic, but this introduces maintenance/drift risk.

A dedicated side-effect-free core surface allows downstream packages to depend on official relayer-sdk utilities without vendoring and without pulling bootstrap dependencies.

Additional context (downstream integration symptom):

• chore(deps): pin @fhevm/mock-utils@0.3.0-4 and @zama-fhe/relayer-sdk@0.4.0 — 0.4.1 incompatible with browser bundling gustavovalverde/zentity#85

Downstream follow-up using this ./core entrypoint:

• refactor(mock-utils): consume relayer-sdk core entrypoint fhevm-mocks#101

What This PR Does

• Adds a dedicated ./core entrypoint for side-effect-free utility imports.
• Wires build + publish outputs for ./core (rollup + API extractor + package exports/files).
• Adds a regression test to ensure importing ./core does not load WASM bootstrap modules and still exposes expected KmsEIP712 behavior.

Scope / Non-goals

• No behavioral changes to existing ./node, ./web, or ./bundle entrypoints.
• No downstream migration in this PR; this is the foundational first step.

Validation

• npx jest --colors --coverage=false src/core.test.ts

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