fix(preview): prevent render processes outliving shutdown

[miguel-heygen]

Summary

Preview shutdown now closes render ownership before the server exits. Active Studio jobs are cancelled and awaited, late child registrations are terminated, and a later CLI run can recover only birth-identity-matched FFmpeg processes that became true OS orphans.

Live --background preview owners remain protected. Recovery requires PPID 1 plus a recorded process identity, and it revalidates that identity before both graceful and forced termination. The original report lacked PID/PPID/command/status evidence, so this fixes the validated shutdown and crash window without claiming that the observed job was definitely orphaned rather than owned by a live background server.

Validation

• Studio render-route disposal: 40 tests passed.
• Engine process tracking and late-registration drain: 11 tests passed.
• CLI preview, Studio server, orphan cleanup, and Windows process controls: 64 tests passed.
• Engine, Studio server, producer, and CLI typechecks passed.
• Changed-file formatting and lint, package-subpath consistency, CLI process ownership, and git diff --check passed.

[github-actions]

Fallow audit report

Found 6 findings.

Dead code (1)

Severity	Rule	Location	Description
major	fallow/unused-export	packages/producer/src/services/render/audioPadTrim.ts:44	Export 'AAC_DELIVERY_TRUE_PEAK_DBFS' is never imported by other modules

Duplication (4)

Severity	Rule	Location	Description
minor	fallow/code-duplication	packages/cli/src/commands/preview.test.ts:387	Code clone group 1 (7 lines, 2 instances)
minor	fallow/code-duplication	packages/cli/src/commands/preview.test.ts:407	Code clone group 1 (7 lines, 2 instances)
minor	fallow/code-duplication	packages/cli/src/utils/orphanCleanup.ts:165	Code clone group 2 (6 lines, 2 instances)
minor	fallow/code-duplication	packages/cli/src/utils/orphanCleanup.ts:190	Code clone group 2 (6 lines, 2 instances)

Health (1)

Severity	Rule	Location	Description
minor	fallow/high-cognitive-complexity	packages/engine/src/utils/processTracker.ts:95	'findOwnedOrphanedFfmpegProcesses' has cognitive complexity 18 (threshold: 15)

Generated by fallow.

[miga-heygen]

Orderly shutdown drain sequence, FFmpeg ownership registry with identity-verified cleanup, and draining latch are all well tested and correct. — Miga