`BuildOptions :: struct #compiler { ...35 methods... }` becomes
`BuildOptions :: struct { }` (an opaque null-sentinel handle) plus 35 free
`ufcs (self: BuildOptions, …) abi(.compiler)` decls in build.sx, each serviced
by a new `comptime_vm.callBuildOptionFn` arm (off `callCompilerFn`). No legacy
`compiler_lib` handler: the names are registered in `bound_fns` with a single
bailing stub only so `weldedCompilerFn` accepts them.
- String lifetime: setters dupe the arg into the persistent `Vm.gpa` (the
Compilation allocator, threaded into both `tryEval` and `runBuildCallback` —
not the per-eval VM arena) and write/append to the threaded `BuildConfig`.
Getters read the field/slice or compute the target predicate from the triple.
- Dispatch routing (Option B): a `#run`/const-init entry that directly calls a
compiler-domain/welded fn (`emit_llvm.entryNeedsVm`) runs on the VM with no
legacy fallback regardless of the `-Dcomptime-flat` gate, so gate-OFF stays
green without a legacy BuildOptions handler (P5.7 retires the legacy interp).
- Mark the 5 `platform/bundle.sx` getter-calling helpers `abi(.compiler)` (they
are comptime-only bundler code; otherwise their now-welded getter calls trip
the runtime-call gate).
- 37 `.ir` snapshots regenerated (std transitively imports build.sx → string-
pool/type-table indices shift); verified `.ir`-only, zero behavior-stream diffs.
BuildOptions `compiler_call` strict bails gone (1609/1614/1615 strict-clean);
1616 now bails on a separate, pre-existing unported bitwise/shift VM gap (`shr`),
to port first in P5.6. 703/0 both gates.
Also sweep the outdated "flat memory" terminology to "comptime/byte-addressable"
across comptime_vm + the plan/checkpoint/CLAUDE docs: the comptime VM is
arena-backed, byte-addressable memory where `Addr` is a real host pointer, not a
flat contiguous address space (flag names `-Dcomptime-flat`/`SX_COMPTIME_FLAT` kept).
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