Image normalization package: decode HEIC/webp/png/jpeg → JPEG (#80)

Prerequisite for seed-packet capture (#81). majordomo's media pipeline is
stdlib-based and cannot decode HEIC or webp — and HEIC is the iPhone camera
default, so the capture feature's very first input would fail on the device
that motivates it. Normalizing at the upload boundary means everything
downstream only ever sees JPEG.

internal/imagenorm.Normalize(reader, opts) decodes any of jpeg/png/heic/webp,
downscales to fit MaxDim (2048, ollama-cloud's limit) on the longest edge with
Catmull-Rom (sharp on the text a packet is mostly made of), and re-encodes JPEG.

CGO stays off: github.com/gen2brain/heic runs libheif as WASM via wazero (pure
Go), golang.org/x/image/webp is pure Go. Both register with image.Decode. The
~5 MB these add only links into the binary when #81 imports this package — it's
not pulled into cmd/pansy yet, so main is unchanged in size for now.

Guards against hostile uploads: input capped at MaxBytes (ErrTooLarge) before a
full read, and the decoded pixel count checked from DecodeConfig BEFORE the
bitmap is allocated, so a small file claiming a huge canvas (a decompression
bomb) is refused up front.

TestNormalizeAllFormats round-trips all four formats to a valid JPEG — its real
job is catching a dropped blank import, where the intuition-breaking failure is
that the COMMON format (png) breaks while the exotic one (heic) still works.
heic/webp fixtures live in testdata (Go has no encoder for them); the webp is a
known-good 16x16 from Python's stdlib test corpus, verified to decode.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

go.mod

@@ -5,9 +5,11 @@ go 1.26.2
 require (
 	gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260718232210-a941f5ff4a3f
 	github.com/coreos/go-oidc/v3 v3.20.0
+	github.com/gen2brain/heic v0.7.1
 	github.com/gin-gonic/gin v1.10.1
 	github.com/samber/slog-gin v1.15.0
 	golang.org/x/crypto v0.36.0
+	golang.org/x/image v0.44.0
 	golang.org/x/oauth2 v0.36.0
 	modernc.org/sqlite v1.34.4
 )
@@ -33,6 +35,7 @@ require (
 	github.com/cloudwego/base64x v0.1.4 // indirect
 	github.com/cloudwego/iasm v0.2.0 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
+	github.com/ebitengine/purego v0.10.1 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.4 // indirect
 	github.com/gin-contrib/sse v0.1.0 // indirect
 	github.com/go-jose/go-jose/v4 v4.1.4 // indirect
@@ -51,14 +54,15 @@ require (
 	github.com/ncruces/go-strftime v0.1.9 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.2 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
+	github.com/tetratelabs/wazero v1.12.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.2.12 // indirect
 	go.opentelemetry.io/otel v1.29.0 // indirect
 	go.opentelemetry.io/otel/trace v1.29.0 // indirect
 	golang.org/x/arch v0.8.0 // indirect
 	golang.org/x/net v0.38.0 // indirect
-	golang.org/x/sys v0.31.0 // indirect
-	golang.org/x/text v0.23.0 // indirect
+	golang.org/x/sys v0.44.0 // indirect
+	golang.org/x/text v0.40.0 // indirect
 	google.golang.org/protobuf v1.34.2 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 	modernc.org/gc/v3 v3.0.0-20240107210532-573471604cb6 // indirect