#4076 refactor(registry): collocate descriptor families and bound the root

Summary

Collocate session/device descriptor families while preserving the synchronous ordered composer. The root falls from 1,834 to 868 lines under the default cap.

Seven files changed; gross diff 2,059 lines in self-contained commits. Exported selectors stay with their snapshot/diff/find consumers; remaining declarations use private literal groups. Source attribution uses existing development-only metadata. Closes #4054; part of #4045. Stack base: #4075 at 7fe445bc68; merge it first.

Local CLI explanations list the declaring family later; their file set is preserved.

Validation

Head: 2b0c79b50cfecef206ea6f465fe8827116b5bc29.

• Rebased after the lower guidance-budget/R66 fixes; conflicts retain the compact owner row. git range-diff shows expected documentation changes. Registry/CLI source is unchanged from reviewed b7d64e4ca8, apart from inherited timeout comments.
• Current guidance tests: five passed; R66 model: ten passed. Guidance totals 39,987 bytes.
• Prior head evidence: 84 ordered descriptors, weight 74 and exports preserved; 107 tests, build, bundle stripping and own-base closure checks passed. Its exact gate passed through package/integration, then failed two provider scenarios. Later stages stayed unrun; the separate package timeout remains unexplained.
• Full validation at this head passed tooling, Fallow and unit against its own base. Receipt .tmp/validation/run-2x6jaj/receipt.json, exactCommit: true. The PR has merged; historical provider failures are not relabeled passed. CI owns additional lanes.