[Export] Align graph export architecture for StableHLO and Minerva

[michalharakal]

Goal

Add Minerva as a secure MCU export backend while aligning the export architecture with the existing StableHLO conversion model.

Architecture direction

Use ComputeGraph as the shared export boundary.

Create a common graph-export workflow where StableHLO and Minerva share:

• graph validation entry points
• conversion context concepts
• registry/factory structure
• structured diagnostics
• result objects suitable for downstream packaging or writing

Keep backend writers separate:

• StableHLO writer emits MLIR text through operation converters.
• Minerva writer emits a Minerva intermediate model, .npz compiler input, compiler artifacts, and a packaged host/firmware project.

Phase-one Minerva scope

• JVM host support
• Sequential MLP models only
• Static shapes only
• Q8 Minerva compiler flow
• Host verification before reporting export success
• Clear secret-handling boundaries for generated artifacts

Out of scope for phase one

• Conv1D export
• BNN export
• In-process Minerva compiler rewrite
• Hardware-in-the-loop CI
• Support for arbitrary MCU targets beyond validated Minerva target configs

Definition of done

• Supported SKaiNET MLP graphs export to a Minerva project bundle.
• The export flow generates compiler input, weights.c, weights.h, manifest, host harness, firmware example, and documentation.
• Unsupported graphs fail before compiler invocation with actionable compatibility diagnostics.
• Host validation runs in CI without MCU hardware.
• Existing StableHLO conversion keeps working while the shared graph-export contracts are introduced incrementally.

[michalharakal]

Closing as implemented. Shared graph-export boundary ships in skainet-compile-core/.../export/GraphExportContracts.kt; the Minerva backend ships in skainet-compile-minerva (MinervaExportFacade, MinervaGraphCanonicalizer, MinervaNpzModelWriter, MinervaCompatibilityValidator, MinervaHostVerificationModels, JVM compiler/packager, secure-MCU + tiny-MLP examples, full test suite) with docs (how-to/minerva-export, tutorials/minerva-getting-started, explanation/minerva-secure-mcu-export). Every phase-one Definition-of-Done bullet is met — bundle export with weights.c/.h + manifest + host harness + firmware example, pre-compiler compatibility diagnostics, host-only CI verification, and StableHLO conversion left intact. Delivered across PRs #712/#717/#719/#721/#725.