A plan for measuring the aggregate case across the reload boundary

HANDOFF-x86-aggregates.md

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+# Handoff — the aggregate case across the reload boundary
+
+Branch `dev-loop`, from `957ba07`. This closes item 4 of `HANDOFF-x86-redef.md`: a redefined function that
+takes and returns a struct, which is the one case the two backends' conventions disagree about and the reason
+`X86.redefinition` exists at all rather than reusing `Emit.redefinition`.
+
+*Status: in progress. This file is written early and on purpose, so that a lane that picks the work up mid-way
+has the plan and not just the diff.*
+
+## The plan
+
+1. A fixture pair, `test/programs/reload-agg.flan` and `reload-agg-v2.flan`, beside the existing `reload.flan`
+   rather than inside it — `reload.flan`'s transcript is shared with the LLVM path and must not move.
+   No `main`; the host is `test/reload_host.c` unchanged, which wants `outer : () -> i64` and a `counter`
+   global. Everything aggregate happens *inside* Flan, between the host's compiled-once call site and the
+   redefined body, because the C boundary is where the two conventions are required to agree and is not what
+   is under test.
+2. Four struct shapes, because SysV treats them in four different ways and `x86.ml` treats all four the same:
+   - `≤16 bytes, all integer` — SysV passes in `rdi`:`rsi` and returns in `rax`:`rdx`; `x86.ml` passes a
+     pointer and returns through a hidden `sret`. Diverges in both directions.
+   - `>16 bytes` — SysV copies the argument onto the stack, which `x86.ml` does not; the *return*, though,
+     is a hidden pointer in the first integer register for SysV too, so that half may well coincide. Worth
+     knowing, and worth not assuming either way.
+   - `{f64, f64}` — SysV classifies both eightbytes SSE and uses `xmm0`:`xmm1`.
+   - `{i64, f64}` — one INTEGER and one SSE eightbyte, so `rax` and `xmm0`, which is a third register pattern
+     again.
+3. Every field weighted by position in the printed answer (`a + 100*b + 10000*c + …`), so that a scrambled
+   field order, a wrong base pointer or a half-loaded eightbyte prints a visibly wrong integer rather than the
+   right one by symmetry. A test whose answer is a symmetric sum of the fields passes under a convention
+   mismatch that merely permutes them.
+4. Run it on both backends: the LLVM `Emit.redefinition` path as the control, and the `X86.redefinition` path
+   as the claim. The same transcript from both is the measurement.
+
+## Open questions recorded rather than asked
+
+- Whether the MEMORY-return half of the matrix genuinely coincides between the two conventions, or only looks
+  as though it should. Measured by the mismatch spike rather than argued.
+
+## Baseline
+
+| | before |
+|---|---|
+| `spike/x86/survey.sh` | 99 MATCH / 0 DIFFER / 0 refused |
+| `spike/x86/cells.sh` | 4/4 ok |
+| `dune test --root .` | exit 0 |
+
+Note in advance: `survey.sh` globs `test/programs/*.flan`, so each new no-`main` fixture adds one to its
+`no-main` skip count. That number moving is the fixtures existing and is not a regression; the MATCH count is
+what must not move.