Add logger newline boundary regression fixture

[h1065153539-create]

Summary

• Adds a focused C fixture for legacy frailbox logger newline boundary behavior.
• Covers messages with no caller newline, one caller newline, multiple trailing newlines, and a message that crosses the internal log buffer limit.
• Keeps the public logger API compatible while ensuring truncated records still end with a newline before the next record starts.
• Documents the fixture target in docs/OPERATIONS.md.

Validation

• make -C frailbox clean logger-newline-fixtures passed.
• make -C frailbox all passed.
• python3 build.py -m frailbox compiled frailbox successfully in the API-sourced worktree; the encrypted diagnostic log records the expected non-git diagnostic commit limitation for this local checkout.

Diagnostic bundle

• diagnostic/build-abef3d5c.logd
• diagnostic/build-abef3d5c.json

Closes #4.

[Soengkit]

Closing this PR because the linked fork issue is only a closed payout tracker for an already submitted upstream PR. This fork is not an active bounty intake or payment authority, and it is not accepting external submissions or payment details here.