feat(renderer): color-by-object mode + object isolation

[sobechestnut-dev]

Backlog candidate (2026-07-23 research pass) · triage against E9 #162.

Add a color-by-object mode (+ optional "isolate object N" toggle). The object channel already exists from the E4 M486 / EXCLUDE_OBJECT work but nothing surfaces it visually.

Data: already parsed — seg.object (1-based index into ir.objects, 0 = none). Verified this session that no color-by-object mode ships.

Approach: a { mode: 'object'; palette; fallback } variant in colors.ts (mirrors the tool mode), plus optionally a control to show only a chosen object. Capability-gate on objects.

Implementation gated by DD/E9 triage per project governance.