feat(kernel): export targets on the exporter port

[JArmandoAnaya]

First phase of the export-targets epic (#784): the kernel now knows what an export target is, and the Exporter port declares them. A target is the model a person will train; it resolves to exactly one exporter, and the catalog every surface will render is derived from what is installed rather than kept anywhere by hand. This phase is kernel and formats only — no route, CLI command, or MCP tool consumes targets yet, and no wire shape changes.

The contract added

• kernel/domain/export_target.py (new): frozen pydantic models — Task, ResizeStrategy, TargetFamily, PreprocessingHints, and ExportTarget with three validators: the name is a lowercase slug, supported_geometries is non-empty, and a recommended strategy requires a recommended size. ResizeStrategy lives here because the hints reference it.
• kernel/ports/exporter.py: the port gains targets: frozenset[ExportTarget] beside its five existing attributes, plus two module-level helpers — validate_targets(exporter), holding every target's supported_geometries within the exporter's own, and resolve_target(installed, name), the pure resolution from a target name to (Exporter, ExportTarget).
• kernel/errors.py: ExportTargetNotFound (carrying the installed target names), ExportTargetConflict, and InvalidExportTarget for the validate_targets refusal.
• Every shipped exporter declares one self-named target — family other, no trainer tasks, geometries equal to its supported_geometries, hints (None, None, trainer_resizes=True, augmentation_common=False) — spelled once in formats/_targets.py. The dummy exporter declares dummy the same way.

Decisions a reviewer should see

• The server error table is updated in this phase. tests/server/test_errors.py holds ERROR_RULES in exact correspondence with kernel/errors.py, so the three new errors are mapped now (404 for the missing target; 500 for the conflict and the defective declaration, opaque) and added to the inventory in docs/content/api.md, on the table's own precedent for errors no route reaches yet. The phase that builds the target routes owns the final status and exposure argument.
• TuSimple's target carries polyline rather than its supported_geometries. That set is empty — everything TuSimple writes arrives degraded — and an empty target geometry set is invalid by design. The subset rule validate_targets enforces therefore cannot hold for an exporter whose entire output is degraded; nothing runs that check against installed plugins yet, and the rule needs a ruling before the registry enforces it at discovery.
• The yolo exporter's self-named target is a placeholder; the dialect work replaces it with the real trainer targets.

Stack

Position 1 of the export-targets stack (#784); based on main.

Checks

Targeted: tests/kernel, tests/formats, tests/architecture, tests/server, tests/cli, tests/mcp, tests/jobs, tests/packaging (all passing), uv run lint-imports (4 contracts kept), uv run mypy src/visionset (clean), uv run ruff check . and ruff format --check . (clean).

Full gate: bash scripts/check.sh — check.sh: PASSED ran=python,frontend,generated,browser skipped=docs ("All checks passed."; the docs group is CI's, and the only docs change here is the api.md code inventory, which tests/server/test_errors.py gates).