refactor(config): split grid-style editor VM into drafts (#118)

[mrcsin]

Problem

The grid-style editor view model declared its ~77 style fields three times — property declarations, a Seed method, and a with-based BuildRecord. A field dropped from BuildRecord compiled fine and silently reverted on save. This is the fifth and final slice of the #118 debloat; it removes the last hand-maintained full-field list in the stack, so field omission stops being representable.

What changed

• Nine draft classes, one per group record (DepthPaletteDraft serves both ReadOnlyCells and DisabledCells). A draft's property initializers are the seed (read from the group record); its Build() constructs the record positionally, so an omitted field is a compile error (CS7036), not a silent revert.
• The parent VM shrinks to draft wiring — ten reactive draft properties, ReplaceDrafts (seed + swap the CanSave subscriptions on load), a BuildRecord of ten Build() calls + Orientation, and CanSave = the numeric-range checks only. SaveCommand, the load-failure path, and error routing are behavior-identical.
• All 77 AXAML binding paths → grouped Group.Leaf form, compile-checked against the window x:DataType (a bad path fails the build, AVLN2000).
• Validate removed from IGridStyleEditorFacade — with typed StyleColor a loaded record is valid by construction, so the seam is Load/Save only.
• Architecture doc rewritten as current-state — the interim slice notes and the obsolete "Known gap" section are gone; the roadmap marks all five slices DONE.

Verification

• dotnet build SemiStep.slnx — 0 warnings; dotnet test — 1727 passed, 0 failed; dotnet format clean.
• The GridStyleEditorViewModel: ~60 fields maintained in three places; sync file I/O on the UI thread #118 thesis, proven — the guard-net non-vacuity scratch checks (recorded, reverted): drop a Build() argument → CS7036; mistype an AXAML path → AVLN2000; miswire a draft initializer → the per-draft round-trip test red; miswire a Build() slot → the perturbation guard red.
• Coverage moved with the structure — nine per-draft round-trips (new <Draft>(fixture.Group).Build() == fixture.Group, DepthPaletteDraft against both palettes) replace the deleted seed guard; the perturbation guard pins the walked leaf count at 77 (asserted, not derived); the re-seed rewiring test proves the subscription swap.
• Behaviour preserved — the untouched suites stay green: the Russian-culture malformed-hex end-to-end message, the owner-routing window tests over the renamed AXAML, SaveThenLoad_DistinctFixture, and GridStyleEditorWindowTests byte-identity. The on-disk YAML is unchanged.
• Review chain: 5 comprehensive + 2 critical re-check + 2 final critical, all OUTCOME: ACHIEVED, no critical/major survived. Fixes: a DraftNumbers guard-dedup, the roadmap Slice-2 status corrected, two null-throw invariant tests, three draft-accessor style alignments, two comment trims.
• Manual test (operator): confirmed.

Note: an unrelated GC-sensitive allocation probe (RecipeGridSurfaceContractTests.AppendStep_PerSurfaceTailAllocation…) flaked on one of three review runs — outside this branch's change surface. Re-run if CI trips it.

Closes #118

Per-task commits before collapse

79f094f docs(plan): record exec branch and verify steps
128ff2c docs(config): trim restatement in grid-style VM comments
1489a03 style(config): align grid-style draft accessors with VM convention
08d3a97 fix: address slice-5 review findings
db41dc8 docs(config): rewrite grid-style doc as current-state
da3342a refactor(config): split grid-style editor VM into per-group drafts
175ea33 feat(config): add grid-style editor per-group drafts
2c1e0f6 refactor(config): drop Validate from grid-style editor facade

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