docs: clarify Quantum forge script support boundary

[williamclay8]

Motivation

The README currently says forge script routes scripted broadcast through the
shared Quantum adapter for supported write shapes and fails closed on unsupported
Quantum script shapes.

Current crates/script/src/lib.rs is stricter: when --quantum is set,
run_script() immediately rejects the command and says forge script --broadcast does not yet support the Quantum adapter path in v1. The rejection
message points builders to individual cast send --quantum and
forge create --quantum invocations instead.

This docs-only clarification keeps early builders from planning multi-step
deployment scripts around forge script --broadcast --quantum before that path
is wired through the native 0x7A pipeline.

Solution

Update the README forge script bullet to say that --quantum intentionally
fails closed in v1 and to name the currently supported alternatives:
cast send --quantum, forge create --quantum, or running without --quantum
against an Ethereum-compatible network.

AI assistance disclosure: I used OpenAI Codex to compare the current README
wording with current raw source and to draft this docs-only clarification. No
local Foundry execution, code generation, signing, deployment, or write
transaction was performed.

PR Checklist

• Added Tests
• Added Documentation
• Breaking changes