m68k: report 68040 write faults in writeback slot 3

[codewiz]

What

The 68040 access-error stack frame builder pushed a faulted write into
writeback slot 2 (WB2S/WB2A/WB2D), and the RTE handler read the pending
writeback status back from that slot.

Real 68040 silicon and the WinUAE/Amiberry MMU core put a normal write
fault in slot 3. See Amiberry src/cpummu.cpp:434:

regs.wb3_status = write ? 0x80 | (ssw & 0x7f) : 0;
regs.wb3_data = val;
regs.wb2_status = 0;

WB2 is reserved for MOVE16 cacheline writes. Frame push order confirmed
at src/newcpu_common.cpp:1442-1455 (WB3D at +$1C, WB3A at +$18, WB3S
at +$0E).

With the data in the wrong slot, a monitor that completes an allowed
store by copying WB3D to WB3A (MuGuardianAngel) reads a zero WB3D and
clobbers the target with 0.

Change

• exceptions.rs: build the write into slot 3 (WB3S/WB3A/WB3D), clear WB2.
• decode.rs: the RTE absorb path reads WB3S/WB3A from +$0E/+$18.
• access_fault_frame_tests.rs: updated to assert the slot-3 layout (8 tests pass).

Scope

This corrects the writeback slot to match the reference; it is not a
fix for the separate access-fault continuation issue tracked in #155, and
no functional repro isolates this change from that bug. It stands on the
frame unit tests.

🤖 Generated with Claude Code

[codewiz]

@codex review

[Copilot]

Pull request overview

This PR corrects the 68040 access-error (format 7) stack frame writeback layout so faulted writes are reported in writeback slot 3 (WB3S/WB3A/WB3D) rather than slot 2, aligning Copperline’s frame contract and RTE continuation logic with real 68040 behavior and the WinUAE/Amiberry reference.

Changes:

• Update 68040 access-error frame construction to populate WB3S/WB3A/WB3D for write faults and clear WB2.
• Update the 68040 RTE “absorb write” path to read WB3S/WB3A at the correct offsets (+$0E / +$18).
• Update access-error frame tests to assert the slot-3 writeback layout and absorb semantics.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
crates/m68k/src/core/exceptions.rs	Builds 68040 format-7 frames with faulted writes placed into WB3* (slot 3) and WB2 cleared.
crates/m68k/src/core/decode.rs	Adjusts RTE continuation logic to consult WB3S/WB3A at the correct frame offsets for write suppression.
crates/m68k/tests/access_fault_frame_tests.rs	Updates assertions and handler behavior to validate the corrected slot-3 layout and RTE absorb behavior.

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