Dissolve std.baseboard's minted BaseboardManufacturer onto Vendor<Domain>

[briansrls]

Summary

std.baseboard re-coined ASRock Rack as a freshly-minted single-variant
BaseboardManufacturer enum (dispatched via a match fn) instead of citing it
on the single Vendor<Domain> authority (DESIGN §3) all other hardware
vendors already ground on (Vendor<Hardware> — see CpuModelCatalogRow,
GpuFacts, DramModuleCatalogRow). Per
docs/plans/extdeps-std-grounding-inventory.md §1/§2 this is an s3-fork.

Because std cannot import extdeps.vendor (the layer DAG points imports
toward std, never away from it), grounding the vendor field forces the
whole baseboard domain shape — BaseboardModel, BaseboardCatalogRow,
ServerBaseboard — out of std/baseboard.dag and into
extdeps/boards/types.dag, matching the existing CPU/GPU/DRAM precedent
where the catalog-row shape lives in extdeps beside its cited vendor rows,
and std keeps only the frameworks (measure, types) those shapes build
on. BaseboardCatalogRow.vendor is now a plain Vendor<Hardware> field
(cited extdeps/vendor/asrock_rack.dag), not a derived-by-match enum —
dropping BaseboardManufacturer and baseboard_manufacturer() entirely.

Consumers updated: dag/extdeps/boards/asrock_rack.dag (catalog row now
carries vendor: asrock_rack), dag/gunbc/fleet_intent.dag (import
repointed), dag/test/claim/bmc_factory_credential_witness_test.dag
(assertion now compares .catalog.vendor against the cited vendor entity
instead of matching the deleted enum).

Test plan

• claim_batch --source-root dag --source-root src/v2 --entry dag/test/claim/bmc_factory_credential_witness_test.dag --function bmc_factory_credential_holds → PASS bmc_factory_credential_holds (green-by-execution, exercises the new vendor field end to end).
• claim_batch ... --entry dag/test/claim/fleet_intent_network_witness_test.dag --function fleet_intent_network_container_to_bmc_allowed → PASS (proves fleet_intent.dag's ServerBaseboard consumers, incl. srv1_baseboard..srv4_baseboard, still resolve and compile clean against the moved module).
• cargo build -p v1-compiler --bin claim_executor / --bin claim_batch → clean.

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