flan/HANDOFF-x86-abi-marker.md

Handoff — the ABI marker symbol, so a crossed pair is refused at dlopen

Branch dev-loop, from 682cb74. This closes the serious finding of HANDOFF-x86-aggregates.md: a redefinition module built by one backend, dlopened into a host built by the other, links and loads and then dies with SIGSEGV at the first call into a redefined function that takes or returns a struct. Nothing refused it, and flan build game.flan --x86 --dev followed by flan reload game.flan changed.flan builds exactly that pair.

The plan

A marker symbol, as that handoff recommended.

• X86.program defines flan.abi.x86; X86.redefinition emits a data relocation against it.
• Emit.program defines flan.abi.llvm; Emit.redefinition emits one against that.
• Both sides gated on dev, so a release build's output is byte-for-byte what it was.
• The two dlopen sites — test/reload_host.c and vendor/agent/flan_agent.c — recognise a flan.abi. failure and say, in a sentence, that the module and the host were built by different backends.
• test/test_reload.ml gains the crossed pair, both directions, asserting the refusal.

flan reload --x86 is judged out of scope; see below.

Status

In progress. This file is written early and updated as the work lands.