[P0] Make the core clip-generation path production reliable

[HeisLazi]

Outcome

A new user can complete the primary InstaClip workflow—from supported input to exported vertical clip—reliably and with understandable recovery when something fails.

Scope

• Use the approved public-edition boundary from the audit issue.
• Stabilize ingestion, analysis, clip selection, editing, rendering, and export.
• Remove or isolate broken/duplicate legacy paths.
• Add deterministic validation, actionable errors, progress states, and retry behavior.
• Add automated coverage for the critical path and representative failure cases.

Acceptance criteria

• One canonical production workflow exists.
• Supported input constraints are clear and enforced.
• Critical stages report progress and actionable failures.
• Interrupted/failed work does not corrupt user state or outputs.
• Automated tests cover the happy path and major failure modes.
• A clean-environment verification command and evidence are documented.

Guardrails

No unrelated redesign, speculative feature work, silent dependency replacement, or public release. Preserve private-source history and obtain human approval for destructive migrations.