Overhaul grid a11y, navigation and rendering

[rkaraivanov]

Screen readers were silent over the grid body: ARIA state lived on scattered attributes and DOM focus never reached the cells. Holding an arrow key re-rendered every visible row per keystroke.

Accessibility

• Implement the full ARIA grid pattern through ElementInternals: roles, row/column counts and indices, aria-sort, aria-selected. The filter row is modeled as a second header row.
• Use roving focus. Navigation and click activation place real DOM focus on the active cell so assistive technology announces it; aria-activedescendant cannot cross shadow boundaries. The virtualizer reclaims focus when scrolling removes the focused cell.

Performance

• Keep the row renderer identity stable across activation. The virtualizer re-renders all rows when renderItem changes, so activation now updates only the two affected rows.
• Derive column track sizes only when the configuration changes and notify context consumers at mutation points, not on every host update.

API and structure

• navigateTo(row, options) replaces positional arguments. Activation without a column keeps the current one instead of clearing it.
• Rename internal tags to the igc-grid-lite-* prefix.
• Defer filter row and editor element definitions until a filterable column renders.
• Route all render-root queries through the DOM controller so other controllers no longer reach into the grid DOM directly.
• Skip filter expressions whose conditions no column resolved.