Add caller-backed PreparedImage and reserve prepared-image region outside kernel fixture

[thanks-cohn]

Motivation

• Batch 31 discovered that the pinned BusyBox artifact requires ~245 candidate pages and that enlarging inline kernel backing is unsafe because it pushes the kernel image into the inherited 0x8040_0000 fixture range. The correct repair is a distinct bounded machine-adapter reservation plus PREPARE-time Sv39 table backing.
• Implement a minimal, general mechanism so PREPARE can reserve backing and table pages atomically without growing the ordinary kernel image or weakening PREPARE/COMMIT atomicity and W+X=0 guarantees.

Description

• Add PreparedImage and PreparedPage to projects/59-bounded-address-space-exec-image/src/bounded_address_space_exec_image.zig to materialize page bytes directly into caller-supplied backing while returning only bounded metadata and capacity errors.
• Extend the freestanding machine path to use caller-backed prepared images by allocating external_prepared_backing and external_prepared_stack in a new linker reservation and invoking PreparedImage.prepare(..., backing) during PREPARE.
• Add a dedicated linker PT_LOAD reservation region at 0x80600000 and PHDR/section changes so prepared-image storage is outside the ordinary image and the inherited fixture window.
• Update PREPARE/COMMIT flow to preflight Sv39 table leaves against reserved backing, materialize the initial stack into reserved storage, replace the live stack only at COMMIT, and preserve W+X=0 and static-musl invariants.
• Update module contracts, port/README/DETAILS/MASTERY text, COMMANDS.md, and add a Batch 31B handoff report documenting the frontier and next action.

Testing

• zig build test-bounded-address-space-exec-image succeeded and unit tests covering inline MaterializedImage and the new PreparedImage behavior passed.
• zig build smoke-bounded-address-space-exec-image succeeded and the focused materialization/safety smoke checks passed.
• zig build install-freestanding-riscv64-morphic-runtime embedding the exact pinned BusyBox artifact completed successfully and produced an executable with a separate reservation PT_LOAD (no load segment now spans the inherited fixture window).
• Repository tooling checks python3 tools/check-command-reference.py --check and the module contract consistency checks passed after updating details.json/port.js.
• Host QEMU-based runtime verification python3 tools/verify-freestanding-riscv64-external-artifact-transport.py --self-test could not run here because qemu-system-riscv64 is not available, so PREPARE/COMMIT and U-mode BusyBox execution were not validated in this environment.
• Repository index generation failed in this environment due to missing node, so aggregate index regeneration was not completed here; the focused module and recipe tests above were used to validate the change instead.

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Codex Task