ci(release): drop macOS arm64 binary build and clarify combined sof-tx modes

[Lythaeon]

Summary

This follow-up PR does two small fixes on top of v1.0.0:

• remove the macos-14 / aarch64-apple-darwin binary build from the release workflow
• clarify in the README that combined submission routing was already supported through sof_tx.mode = "hybrid" and sof_tx.mode = "custom"

Motivation

The current release workflow fails on the macOS 14 arm64 binary build due to the known upstream sof Linux-gated import issue in that target environment. Removing the arm64 build unblocks the release pipeline without changing the runtime itself.

Separately, the README still under-documented combined submission routing even though the implementation already supported it.

Risk Notes / Regressions Considered

• this removes one release artifact target (macos-arm64) rather than attempting a risky workaround in the release branch
• runtime behavior is unchanged
• documentation change only clarifies existing behavior

Validation Performed

• inspected the failing release target matrix entry
• verified the README clarification matches the existing sof_tx mode behavior already present in the codebase