Fixed-address tests contend across crates (wrong, then fixed, now closed)

The diagnosis was wrong and cost four gate runs. It was never about addresses, and orbistoun-mem was never involved.

Two tests in orbistoun-abi share one static SEEN: [AtomicU64; 6] - the array the emitted machine code writes its arguments into - and cargo runs them on parallel threads within one binary. Each could see the other's arguments.

The failure said so all along: "argument 0 (rdi) arrived wrong - left: 3". Three is not a corrupted address. It is the_call_can_be_made_repeatedly on its third round, which writes exactly 3 into that slot. Reading the value rather than the shape of the failure would have found this the first time.

Fixed by serialising the two tests on a mutex. Eleven passes, five runs running.

The lesson worth keeping: "intermittent under a full workspace run" was assumed to mean cross-crate contention, and that assumption survived three sightings because it was plausible and nobody checked it against the number in the message. An entry recording a wrong cause is worse than one recording an unknown cause - it stops anybody looking.

The half of this inside one crate is now fixed, and it bit exactly as predicted. A test added to orbistoun-mem picked TEST_BASE + 0x300_0000 by reading the file and choosing a gap - and a test three functions further down was already using it. Tests in a binary run on parallel threads, so the two raced and one failed about one run in ten: often enough to be real, rare enough to be dismissed. stack.rs hands out bases from a counter now, so no test picks an address at all.

The cross-crate half is now closed (D324). orbistoun-mem::test_bases holds one range per crate and hands out addresses inside it, so nobody picks a number at either level - not a test within a binary, and not a crate within the workspace. A test asserts the ranges are distinct, because the table is exactly the sort of thing somebody appends to in a hurry.

It was closed because the hazard bit a third time, in the way the entry above predicts almost word for word: four new tests in orbistoun-thunk took the shipped base and raced, and the first repair picked offsets by hand - which is the documented way to reintroduce it.