feat: add modelsdk — auto-generated type-safe model SDK

[engalar]

Summary

Add a new modelsdk/ package — a fundamentally better architecture for reading and writing Mendix MPR files. This is additive only (no existing code modified) and coexists with sdk/ to enable gradual migration.

Why this is a better solution

The current sdk/ layer has five structural problems that can't be fixed incrementally:

Problem	Current sdk/	New modelsdk/
Coverage	~480 hand-written types, 5-10 domains	1,500+ auto-generated types, 53 domains
Type dispatch	Manual if/switch in 52 parser files	Auto-registered init() → DefaultRegistry
Dirty tracking	None — full rewrite on every save	Bitmap per element + container chain propagation
BSON roundtrip	Unimplemented fields silently dropped	Raw BSON preserved; unknown types survive read/write
Adding new types	Touch 5+ files, write parser+writer by hand	Update supplements.json, re-run codegen

Architecture highlights

• Property abstraction — Primitive[T], Part[T], PartList[T], Enum[T], ByNameRef[T] unify field access with lazy init and dirty tracking
• Three-branch encoder — self-dirty (full rebuild), child-dirty (selective rebuild), clean (zero-copy pass-through)
• Lazy decode — InitFromRaw() parses BSON on first access; unaccessed fields cost nothing
• Version metadata — every property records introduced/deleted/public version info from Mendix metamodel
• Reference registry — auto-generated cross-domain relationship tracking (ByName/ById/ByNameList)
• Unknown type safety — unrecognized $Type values decode to generic element.Base preserving raw bytes, so the document round-trips without data loss

What's included

Package	Purpose
modelsdk/codec/	BSON encoder/decoder with type registry
modelsdk/element/	Element interface with dirty bitmap
modelsdk/property/	Generic property types with lazy init
modelsdk/gen/ (53 packages)	Auto-generated domain types
modelsdk/mpr/	MPR v1/v2 reader/writer
modelsdk/widgets/	Widget template handling
modelsdk/meta/	System module metadata
cmd/modelsdk-codegen/	Code generator from TS SDK reflection data
internal/codegen/dtsparser/	TypeScript SDK .js file parser
internal/codegen/emitter/	Go code generation templates

Test plan

• go build ./modelsdk/... compiles clean
• go build ./cmd/modelsdk-codegen/... compiles clean
• go test ./modelsdk/... — 7 packages pass (codec, element, property, version, widgets, mpk, model)
• No existing tests broken (additive change only)

🤖 Generated with Claude Code