fix: use registered ext coders for named non-struct types (#55)

[youdie006]

Repro

Register an ext coder for a named non-struct type (as commonly used for Go "enums"):

type Role uint8 // ext code 0x02

• msgpack.Marshal(Role(1)) returns 0x01 (plain fixint) instead of the fixext frame d4 02 01.
• msgpack.Marshal([]Role{1, 2}) returns an array of plain ints (92 01 02) instead of ext frames (92 d4 02 01 d4 02 02).

Because the value is never encoded as an ext, Unmarshal cannot recover it either, so the round-trip is lossy.

Root cause

AddExtCoder / AddExtEncoder accept an ext.Encoder whose Type() may be any reflect.Type, but the ext registry was only consulted in the struct dispatch (calcStruct / writeStruct, and setStruct on decode). The general dispatch (calcSize / create on encode, decode on decode) switches purely on reflect.Kind, so an ext coder registered for a named non-struct type is silently ignored at the top level, inside slices, and as a struct field.

This is a self-inconsistency: time.Time works through the same AddExtCoder API only because its kind is Struct; an equally-valid ext coder for a named int is dropped. The public API accepts any reflect.Type without restriction, so ignoring it is a correctness bug, not a documented limitation.

Fix

Consult the ext registry by rv.Type() at the top of the general dispatch functions, mirroring exactly what the struct path already does:

• encode: calcSize -> CalcByteSize, create -> WriteToBytes (guarded by rv.IsValid() for omitted-field zero Values).
• decode: decode mirrors setStruct's ext dispatch, so a fixext frame is restored into a named non-struct target.

When no ext coder matches, control falls through to the existing kind switch, so non-ext types, the struct path, and time.Time are unaffected.

Tests

• internal/encoding/ext_test.go: asserts a named uint8 ext coder produces the fixext frame at the top level and inside a slice (not plain fixints).
• ext_nonstruct_test.go: full Marshal -> Unmarshal round-trip (top level and slice) proving the value is restored.

go test ./..., gofmt -l ., and go vet ./... are all clean.

Reported by @Beefster09.

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This change was made with AI assistance and reviewed by me before submission.

[Copilot]

Pull request overview

Fixes a correctness gap in the ext-coder dispatch so that ext coders registered for named, non-struct Go types (e.g., enum-style type Role uint8) are honored during general encoding/decoding paths, restoring lossless round-trips.

Changes:

• Encode: consult the ext encoder registry by rv.Type() before the kind switch in calcSize and create.
• Decode: consult the ext decoder registry before the kind switch in decode, mirroring the struct-path ext handling.
• Add tests covering named non-struct ext encoding and end-to-end Marshal/Unmarshal round-trip (top-level + slice).

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
internal/encoding/ext_test.go	Adds a focused internal test ensuring a named uint8 type uses fixext frames (top-level + slice).
internal/encoding/encoding.go	Adds ext-dispatch to the general encode size/write paths (not just the struct path).
internal/decoding/decoding.go	Adds ext-dispatch to the general decode path (not just the struct path).
ext_nonstruct_test.go	Adds an end-to-end public API test for Marshal/Unmarshal round-trips of named non-struct types.

Suppressed comments (1)

internal/encoding/encoding.go:272

• Same as calcSize: this ext fast-path runs before the kind-based nil handling. If an ext encoder is registered for a pointer/slice/map type, nil values will be routed to WriteToBytes instead of encoding as nil, which is a behavior change vs the existing switch and may be surprising/unsafe.

	if rv.IsValid() {
		for i := range extCoders {
			if extCoders[i].Type() == rv.Type() {
				return extCoders[i].WriteToBytes(rv, offset, &e.d)
			}

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