Brittleness #9: global topology registry (per-compilation isolation)

Topologies are registered in process-global state at parse time, so a topology declared while compiling one program leaked into the next when several are compiled in one process (test binary, build server, LSP).

Partially landed (5e43900): arch::reset_topology_registry() restores the built-in baseline; the driver calls it at the start of each compilation. Closes the sequential-reuse leak. Unit test reset_clears_custom_topologies_but_keeps_builtins; additive registry tests share a mutex.

Still open: a fully thread-isolated per-compilation registry (concurrent in-process compiles still share one global) — thread a registry through parser → typechecker → codegen instead of the LazyLock global.

Tracked in docs/discussions/implementation_plans/brittleness_hardening.md (item 9).

Status: partially landed; remainder open.

Start with docs/discussions/implementation_plans/brittleness_hardening.md item 9, then trace the driver and the parser → typechecker → codegen path around arch::reset_topology_registry() and the LazyLock registry. The remaining work is complete when concurrent in-process compilations use isolated topology registries without sharing custom declarations, while built-ins remain available.