feat: support explicit registry-only Trino deployments

[benben]

Summary

• Add explicit DUCKGRES_TRINO_REGISTRY_ONLY=true support for deployments with registered cells and no legacy coordinator. The default still rejects a missing legacy URL; registry-only mode requires a valid registry and rejects mixed configuration.
• Require explicit initial placement before enablement in registry-only mode. Preserve existing ownership and legacy defaults; never assign an arbitrary registered cell or reinterpret legacy rows.
• Expose safe unassigned status and an explicit cell picker on operational Trino pages. No warehouse migration or Gateway routing changes.

Validation

• Red/green startup, admin and enablement regressions.
• just test-trino, just test-controlplane-k8s, just test-mw-fixtures, just ui-test (195 tests), and just lint pass locally.
• Independent adversarial review completed.
• The first live run passed legacy compatibility, initial placement, tenant isolation and green hydration. Its registry-only phase found a fixture Service-convergence race after restart; bounded registry checks and behavioral regression tests now cover that transition.
• The isolated Trino CI lane now restarts its control plane in registry-only mode after existing legacy/multicell checks, verifies both registered backends, legacy-owner rejection, and absence of the legacy bundle route, then restores its fixture.

Draft until the extended live Trino CI lane passes. This does not alter a shared deployment.

[github-actions]

Test Impact Plan

Deterministic summary of how this PR changes tests, CI runners, and coverage-risk signals.

Summary

Area	Added	Changed	Deleted
Test files	1	7	0
E2E/journey files	0	0	0
Workflow files	0	0	0

Signals

• Test cases: +9 / -0
• Assertions: +51 / -3
• Skips or known failures added: 0
• Workflow continue-on-error added: 0
• Workflow path filters added: 0
• Test commands removed from justfile: 1
• E2E/journey retry lines added: 0

Coverage risk: likely reduced

Warnings

• Test command removed (likely reduced)
  • 1 test command line(s) removed from justfile

[bill-ph]

Reviewed the registry-only Trino deployment path, including strict configuration validation, explicit initial-cell admission before enablement, fail-closed handling for unassigned/unknown ownership, and the accompanying admin/E2E regression coverage. I found no P0 blocker requiring a code change. The PR is appropriately kept as draft pending the extended live Trino CI lane; current CI is still in progress.\n\n— Robo Bill