ref(classify): separate core classifier from legacy backport

Move legacy-specific field translations out of the core classifier into
LegacyBackport(), called by webicached before writing the JSON cache.

Core classifier now outputs canonical values:
- Go dist arm → armv6 (correct per GOARM default)
- ffmpeg Windows .gz → .gz (correct file extension)

LegacyBackport remaps for Node.js compat:
- Go dist armv6 → arm (production keeps raw API value)
- ffmpeg Windows .gz → exe (production releases.js override)

sass armv6→armv7 stays in classifier (Dart Sass genuinely targets ARMv7).

cmd/webicached/main.go

@@ -316,6 +316,8 @@ func (wc *WebiCache) refreshPackage(ctx context.Context, pkg pkgConf) error {
 	if err != nil {
 		return fmt.Errorf("classify: %w", err)
 	}
+	// Legacy backport: translate canonical values to what Node.js expects.
+	classifypkg.LegacyBackport(name, assets)
 	classifyDur := time.Since(classifyStart)
 
 	// Step 3: Write to fsstore.

internal/classifypkg/classifypkg.go

@@ -22,7 +22,6 @@ import (
 	"github.com/webinstall/webi-installers/internal/rawcache"
 	"github.com/webinstall/webi-installers/internal/releases/bun"
 	"github.com/webinstall/webi-installers/internal/releases/chromedist"
-	"github.com/webinstall/webi-installers/internal/releases/ffmpeg"
 	"github.com/webinstall/webi-installers/internal/releases/fish"
 	"github.com/webinstall/webi-installers/internal/releases/gitea"
 	"github.com/webinstall/webi-installers/internal/releases/flutterdist"
@@ -151,8 +150,6 @@ func TagVariants(pkg string, assets []storage.Asset) {
 	switch pkg {
 	case "bun":
 		bun.TagVariants(assets)
-	case "ffmpeg":
-		ffmpeg.TagVariants(assets)
 	case "fish":
 		fish.TagVariants(assets)
 	case "git":
@@ -271,6 +268,54 @@ func ApplyConfig(assets []storage.Asset, conf *installerconf.Conf) []storage.Ass
 	return out
 }
 
+// LegacyBackport translates canonical classifier output into field values
+// the Node.js legacy cache expects. The core classifier uses correct,
+// canonical values (armv6, .gz, etc.); this function remaps them for
+// backward compatibility with the production resolver.
+//
+// Call this after Package() when writing to the legacy JSON cache.
+// The new Go resolver will use canonical values directly.
+func LegacyBackport(pkg string, assets []storage.Asset) {
+	switch pkg {
+	case "go":
+		legacyBackportGo(assets)
+	case "ffmpeg":
+		legacyBackportFFmpeg(assets)
+	}
+}
+
+// legacyBackportGo remaps Go dist's arm classification.
+//
+// The Go dist API uses bare "arm" (GOARM default = 6). The classifier
+// canonicalizes this to "armv6". Production keeps the raw "arm" value
+// because normalize.js doesn't run on Go dist assets. The legacy resolver
+// expects "arm" for these assets.
+func legacyBackportGo(assets []storage.Asset) {
+	for i := range assets {
+		if assets[i].Arch == "armv6" {
+			assets[i].Arch = "arm"
+		}
+	}
+}
+
+// legacyBackportFFmpeg remaps Windows gzipped executables.
+//
+// ffmpeg-static publishes bare executables for Windows. The .gz files
+// are gzip-compressed bare executables (not tar archives). Production's
+// ffmpeg/releases.js hardcodes rel.ext = 'exe' for Windows assets.
+// The legacy resolver needs "exe" to serve these correctly.
+func legacyBackportFFmpeg(assets []storage.Asset) {
+	for i := range assets {
+		if assets[i].OS != "windows" {
+			continue
+		}
+		switch assets[i].Format {
+		case ".gz", "":
+			assets[i].Format = ".exe"
+		}
+	}
+}
+
 // ReadAllRaw reads all non-directory, non-underscore-prefixed files from
 // the active generation of a rawcache directory.
 func ReadAllRaw(d *rawcache.Dir) (map[string][]byte, error) {
@@ -883,9 +928,7 @@ func normalizeGoArch(goarch string) string {
 	case "386":
 		return "x86"
 	case "arm":
-		// Go API uses bare "arm" — keep as-is to match production.
-		// The resolver handles arm compatibility (armv7→armv6 fallback).
-		return "arm"
+		return "armv6"
 	case "ppc64le":
 		return "ppc64le"
 	case "ppc64":

internal/releases/ffmpeg/variants.go

@@ -1,29 +0,0 @@
-// Package ffmpeg provides variant tagging for ffmpeg-static releases.
-//
-// ffmpeg-static publishes bare executables for Windows. The .gz files
-// are gzip-compressed bare executables (not archives). Production
-// classifies these as ext "exe".
-package ffmpeg
-
-import (
-	"github.com/webinstall/webi-installers/internal/storage"
-)
-
-// TagVariants fixes Windows asset extensions for ffmpeg.
-//
-// Windows .gz files contain a gzipped bare executable, not a tar archive.
-// Production treats these as ext "exe". Bare Windows files (no extension)
-// are also executables.
-func TagVariants(assets []storage.Asset) {
-	for i := range assets {
-		if assets[i].OS != "windows" {
-			continue
-		}
-		switch assets[i].Format {
-		case ".gz":
-			assets[i].Format = ".exe"
-		case "":
-			assets[i].Format = ".exe"
-		}
-	}
-}