Warn when masterfeat expansion silently drops unobtainable feats (#31)

`classFeatExpansionCanUseFeat` excludes `ALLCLASSESCANUSE=0` feats from class feat table masterfeat expansion. When such a feat also has no `MinLevelClass`, no class can ever obtain it — a data bug that previously failed silently (Spell Focus vanishing from the wizard table, Favored Enemy from ranger, proficiencies from commoner).

New `validateClassFeatMasterfeatAccessibility` warns for exactly that case; exclusions with an explicit `MinLevelClass` (deliberate class restrictions, e.g. fighter weapon specialization) and `ALLCLASSESCANUSE=1` feats stay silent. No keys are hardcoded — everything is derived from the datasets.

Verified: new test + full `go test ./internal/topdata/` pass; against live sow-topdata data it flagged 36 real drops (all fixed by the companion sow-topdata PR, after which it reports 0).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #31
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

internal/topdata/topdata.go

@@ -268,6 +268,7 @@ func ValidateProject(p *project.Project) ValidationReport {
 	validateNativeLockAllocation(dataDir, p.EffectiveConfig().TopData.RowGeneration, &report)
 	validateGeneratedFeatFamilies(dataDir, &report)
 	validateClassSpellbooks(dataDir, &report)
+	validateClassFeatMasterfeatAccessibility(dataDir, &report)
 	validateItempropsRegistryGraph(dataDir, &report)
 	if _, err := os.Stat(statePath); err == nil {
 		report.Files++