refactor(config): type grid-style colors, fold validation (#118)

[mrcsin]

Problem

The grid-style config layer restated its key structure three times: 53 raw #RRGGBB strings on the group records, a 242-line color-enumerating GridStyleValidator, and the mappers' string plumbing. "Is this hex valid" was asked far from where the string is parsed, and a color was an untyped string everywhere it flowed. Slice 4 of the #118 debloat types the colors and moves validation to the one place hex is parsed.

What changed

• StyleColor — a readonly record struct in Core (A/R/G/B channels, Avalonia-free) with Parse/TryParse/ToString. ToString emits #RRGGBB when opaque and #AARRGGBB otherwise, uppercase; Parse accepts exactly 6 or 8 hex digits and rejects the #RGB/#ARGB shorthand the file header wrongly advertised (the validator always rejected it too).
• 53 color fields → StyleColor across the group records. PaletteBrushFactory and the view model convert channel-wise through a new StyleColorConversions; HexColor is deleted.
• Validation folded into the load mapper — GridStyleMapper.Map now returns Result<GridStyleOptions> and accumulates per-key errors: the same typed errors the deleted GridStyleValidator emitted, keyed by the same section path + key name, aggregated (not fail-fast), in the same emission order. Both call sites (ConfigFacade startup, GridStyleEditorFacade) map through the one seat; the facade's Validate is a deliberate vacuous pass-through kept until slice 5. GridStyleValidator (242 lines) is deleted.
• Header + doc fixes — both shipped config headers (MBE, MOCVD) drop the #RGB/#ARGB shorthand claim (comment-only; the writer copies headers forward, so the lie would have persisted); the architecture doc's load-pipeline description follows the new one-pass mapper.

Out of scope by design (slice 5): the view-model split, grouped AXAML, and trimming the vacuous Validate off the interface. The DTOs keep their string properties, so the on-disk YAML is unchanged.

Verification

• dotnet build SemiStep.slnx — 0 warnings; dotnet test — 1716 passed, 0 failed; dotnet format clean.
• Error identity preserved — the error tests were retargeted from GridStyleValidator.Validate(dto) to GridStyleMapper.Map(dto) with byte-identical assertion bodies; the untouched localization + Russian-culture end-to-end message test pass unchanged. Diffed the deleted validator against the new mapper line-by-line: same error types, same section/key strings, same emission order.
• Aggregation + the untested identity path proven by scratch checks (reverted) and now a committed regression test Validate_NullCells_FailsWithExactlyThreeSectionErrors — a null Cells yields exactly colors.cells.execution/readonly/disabled and no bare colors.cells error (a path the entire prior suite never exercised).
• Lossless round-trip — StyleColor.Default is byte-equal to the old strings; SaveThenLoad_DistinctFixture (record→dto→yaml→dto→record) green. The one behavior change is hex case normalizing to uppercase on save, which the editor already does by rewriting the whole file.
• Review chain: 5 comprehensive + 2 critical re-check + 2 final critical, all OUTCOME: ACHIEVED, no critical/major survived. Fixes applied: 5 doc self-contradictions, the null-Cells regression test, 3 StyleColor edge cases, the hand-rolled hex parse simplified to byte.TryParse(AllowHexSpecifier), 2 comment trims.
• Manual test (operator): confirmed.

Slice 4 of #118 (stays open). Next: slice 5 splits the view model into per-group drafts + grouped AXAML, and closes #118.

Per-task commits before collapse

a2cdd3a docs(plan): record exec branch and verify steps
1f7792f docs(config): trim restatement in grid-style mapper comments
cfd5963 fix: address slice-4 review findings
b382fe2 docs(config): correct grid-style color-format header claims
00c88f5 refactor(config): fold grid-style validation into load mapper
f7fd631 refactor(config): type grid-style colors as StyleColor
b7dab17 feat(config): add StyleColor value type

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