feat: add headerFilter() to table columns for inline filter components

[leek]

Summary

Adds a new headerFilter() method to table columns, allowing any BaseFilter subclass to render inline under the column header. This provides a richer alternative to searchable(isIndividual: true) — instead of a plain text search input, users can attach select dropdowns, date pickers, custom multi-field schemas, and more directly to column headers.

Examples

// Select filter under column header
TextColumn::make('status')
    ->headerFilter(
        SelectFilter::make('status')
            ->options(OrderStatus::class)
            ->native(false)
    )

// Custom range filter with side-by-side fields
TextColumn::make('price')
    ->headerFilter(
        Filter::make('price')
            ->columns(2)
            ->schema([
                TextInput::make('min')->numeric()->placeholder('Min'),
                TextInput::make('max')->numeric()->placeholder('Max'),
            ])
            ->query(fn (Builder $query, array $data) => $query
                ->when($data['min'] ?? null, fn ($q, $v) => $q->where('price', '>=', $v))
                ->when($data['max'] ?? null, fn ($q, $v) => $q->where('price', '<=', $v))
            )
    )

Design

• Header filters use a separate Livewire state ($tableHeaderFilters) and form (tableHeaderFiltersForm) — fully isolated from panel filters
• Always live — applies immediately on change, regardless of deferFilters()
• Filter field labels are auto-hidden (the column header serves as the label)
• Multi-field schemas support ->columns() for side-by-side layout with dense gap
• Fully backwards-compatible — searchable(isIndividual: true) is unchanged
• Supports filter indicators, reset, session persistence, and records() data sources

Fixes

Fixes #18108
Fixes #15714
Fixes #8879
Fixes #6842

Test Plan

• Column with SelectFilter renders a select under the header and filters records by exact match
• Column with custom Filter schema renders inline and applies the custom query
• searchable(isIndividual: true) still works unchanged
• Header filter indicators appear and can be removed
• resetTableFiltersForm() resets header filters
• Hidden columns' header filters don't apply to the query
• Hidden filters (->hidden()) don't render in the header row
• Non-query records() tables receive headerFilters in the callback
• PHPStan passes with no new errors