refactor(config): nest grid-style options into group records (#118)

[mrcsin]

Problem

The flat 78-arity GridStyleOptions record forced a wide flatten/unflatten surface across the grid-style stack: two mappers, a color-enumerating validator, and a ×3 view-model mirror (property declarations, Seed, BuildRecord) each had to name every one of the 78 fields. A field surfaced in one block but dropped from another compiled fine and silently misbehaved. Slice 3 of the #118 grid-style debloat replaces the flat record with a nested shape so each reader touches only the group it needs.

What changed

GridStyleOptions is now a one-level nested root composing 10 group records + root Orientation:

• GridStyleFonts, GridStyleLayout, SelectionColors, ChangedCellColors, DepthPalette (one type reused for ReadOnlyCells and DisabledCells), ExecutionPalette, StatusBarStyle, ValidationPanelStyle, ChromeColors.
• Both mappers rebuilt as per-group construction, preserving the exact fallback regimes: whole-DTO-null → return Default; ?? defaults.<group>.<field> for the loose groups (fonts/layout/selection/changed/grid_line/status_bar/validation_panel/chrome); a no-fallback ! chain for the three validator-guaranteed cell palettes (readonly/disabled/execution); Orientation via Parse. colors.grid_line folds into Chrome.GridLine; colors.cells.changed/changed_selected into ChangedCellColors — the two DTO cross-sections the one-level nesting absorbs into the mapper.
• All grid and style-editor consumers read the nested paths; the view model's Seed/BuildRecord rebuild each group positionally (a dropped field is now a compile error, CS7036).
• The slice-1 silent-drop guards are adapted to the nested record — a hand-maintained VM-property→nested-path map plus a typed recursive leaf walk that keeps root Orientation a first-class leaf — and their non-vacuity re-proven by scratch checks.

Deliberately out of scope (later slices): colors stay string (StyleColor is slice 4); the view model's public property surface is unchanged (the split is slice 5). The on-disk YAML, the DTOs, and GridStyleValidator are byte-identical.

Verification

• dotnet build SemiStep.slnx — 0 warnings, 0 errors; dotnet test — 1687 passed, 0 failed; dotnet format clean.
• Default is byte-equal to the old flat Default, so the nested record round-trips losslessly through the unchanged file format (SaveThenLoad_DistinctFixture, record→dto→yaml→dto→record, green).
• The adapted guards were proven to still bite by scratch checks: a BuildRecord cross-wire → BuildRecord_PerturbingEachProperty… RED; a dropped Seed line → Seed_PopulatesEverySurfacedProperty… RED; a dropped GridStyleDtoMapper line → SaveThenLoad_DistinctFixture RED.
• Review chain: comprehensive (5 agents) → smells → comment audit → critical (2 agents), all OUTCOME: ACHIEVED, no critical/major at any point. Fixes applied: 2 stale with-mechanism comments corrected, 10 restatement banners removed.
• Manual test (operator): confirmed. The style-editor Save that rewrote grid_style.yaml during testing was discarded (config file unchanged on the branch).

Slice 3 of #118 (stays open). Next: slice 4 introduces StyleColor and folds the validator's per-key checks into the mapper.

Per-task commits before collapse

d987331 docs(plan): record exec branch and verify steps
6c589e2 test(config): drop restatement banners in grid-style guard map
5235d36 docs(config): correct grid-style BuildRecord comments
7b05310 docs(config): note grid-style record is now nested
5d0d652 refactor(config): nest GridStyleOptions into group records

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