make build cross-compiles for the selected platforms; dist ships every supported one #598

Amended on delivery, 2026-08-24. The capability shipped in full; the default did not. make build
defaults to the current platform, and PLATFORM=all selects every supported one — measured warm, the full
matrix costs 47.5s against 16.5s for the host alone, so building six platforms on every local build was a ~3x
tax paid by everyone to benefit the rare cross-compile. make dist defaults to all platforms, because a
release is not a selection. The text below is the 2026-08-21 ruling as written; read "by default" as
"on request" throughout.

Delivered by #654 (with #653 removing the blocker). Final surface:

make build                                   # this machine
make build PLATFORM=linux/amd64
make build PLATFORM=all
make dist                                    # every supported platform
make dist  PLATFORM="linux/amd64 darwin/arm64"

Chore (USER, 2026-08-21). Building on Windows is a pain — no GNU make, no maintained build
environment — and with the platforms at (near) feature parity, the build should come to the machine
instead: make produces every platform's binaries from one host, and installing anywhere is a copy.

Why it works

The codebase is pure Go (zero import "C" — verified 2026-08-21), so the full matrix cross-compiles
from any host with GOOS/GOARCH alone; no per-platform toolchains, no build VMs. The Makefile already
sweeps the supported GOOS list (vet-all, lint-all, build-all) — but build-all is a compile-check
that emits nothing. The missing piece is the emitting variant with the ldflags stamp.

The shape

1. Matrix: darwin/arm64, linux/amd64, windows/amd64, windows/arm64 — the last is the native
   fit for Parallels on Apple Silicon (Windows-on-ARM).
2. Binary set: the products — lore, writ, star, devlore-test. Dev-side tools
   (devlore-docs, devlore-index, devlore-inventory, scenario) stay host-only.
3. Layout: build/<goos>-<goarch>/<binary>[.exe], each built with the standard ldflags
   (version, commit, date).
4. make build = full matrix by default (ruled), with a PLATFORMS= override to restrict — the
   relief valve for the inner dev loop's extra link time. Generators run once, host-side, shared by all
   targets.
5. Tests stay native per platform in CI — cross-compiled binaries cross-run nowhere, and the CI matrix
   already covers execution.

Relations

Forces #572 (the stale provider-gen groups that broke cross-GOOS make build) — subsumed or fixed
en route. Sits beside the platform sweep of #373 phase 1b, which supplied the GOOS list this reuses.

Epic: #453