Implement Request Context

[suraciii]

Summary

• Add immutable Request Context helpers.
• Carry validated scalar values through local and forwarded Calls.
• Preserve exact integer values and reject invalid wire data.
• Add tests for isolation, lifecycle, State, Reminders, cancellation, and forwarding.
• Update the design Gap and error rules.

Verification

• make test
• make sim
• make gen
• make net
• make lint
• make ci

Scope

This PR does not add Call Filters, reentrant Calls, or Request Context persistence.

Closes #94