feat: Semantic diff and three-way merge for .tm7 deconfliction

[Hacks4Snacks]

feat: semantic diff and three-way merge for .tm7 deconfliction

What & why

.tm7 is DataContractSerializer XML which is verbose and order-sensitive so git's line-based
merge corrupts it (conflict markers break deserialization) and its diffs are unreadable.
This adds a structural, identity-aware diff and merge keyed by the stable per-node GUID every
model already carries, exposed via the CLI, wired into git, and surfaced in Studio so .tm7
becomes reviewable and mergeable in normal branch/PR workflows.

Highlights

• Diff/merge core (Editing): geometry-free, GUID-keyed ModelSnapshot/ModelDiff and
  ModelMerge. Non-overlapping edits combine; genuine conflicts (same-property divergence,
  delete/modify, dangling connector) resolve to ours and are reported, the written model is
  always valid, never marker-laced. Falls back to a two-way merge when the common ancestor
  isn't available.
• CLI: tmforge diff <a> <b> [--json] and tmforge diff --textconv (readable git diffs);
  tmforge merge <base> <ours> <theirs> shaped as a git driver (writes a <path>.conflicts.json
  sidecar + non-zero exit on conflict); tmforge git-setup [--global] [--print] wires the
  diff/merge drivers with no committed .gitattributes required.
• Git integration: .gitattributes (*.tm7 diff=tmforge merge=tmforge) + textconv/merge-driver
  config; fully opt-in, so git behaves exactly as before until configured.
• Engine/API/WASM: shared EngineService.Merge (nullable base -> two-way) behind
  POST /v1/model/merge and the in-browser WASM Engine.Merge; OpenAPI v1.json regenerated.
• Formats: tmforge-json read now preserves GUID element and surface ids, so identity
  survives the round-trip and pages align across files (also makes diff meaningful on .tmforge.json).
• Studio: a Merge toolbar button opens a resolve modal — upload ours/theirs (base optional),
  pick ours-or-theirs per conflict, then load onto the canvas or download .tm7; app-styled file pickers.
• Tests: diff / snapshot / merge unit tests (Editing), CLI diff / merge / git-setup tests, engine
  merge tests, and id-preservation coverage; validated with real git merge and in-browser WASM smokes.
• Docs: cli-reference, studio-guide, and README updated.