Return typed errors for invalid linear job builder state

[bnomei]

Context

Current shorthand builder methods should become fallible where call order can be invalid. Removing the panic is acceptable even as a breaking API cleanup.

Tasks

• Identify builder methods that can panic due to invalid call order.
• Return typed invalid-builder-state errors from those paths.
• Update examples and tests for the fallible API.

Acceptance criteria

• Invalid builder order returns a typed error.
• No user-triggerable panic remains in shorthand builder paths.
• Examples show the new error handling.

Notes

• Generated from the project improvement research pass.
• Keep implementation discussion repository-relative; do not include local machine paths or private setup details.