Feat/canonical text rendering contracts

[AIEraDev]

Architecture-First Delivery

Complete every applicable item before review. If an item does not apply, write
N/A and why.

• Root cause and owning subsystem:
• Single authoritative state and execution flow:
• Affected consumers: editor / native preview / browser preview / export /
  serialization-project loading
• Cross-runtime verification: for each affected consumer, test and result:
• Focused regression coverage:

Transition layer (if any)

• Owner and bounded scope:
• Remaining consumers of the old path:
• Tracking issue or retirement milestone:
• Checkable removal condition:

Native playback or performance work (if any)

• Native test environment: OS, device, sample rate/channels, representative media:
• Measured result: before/after, or diagnostic trace including installed/active
  clips, mixer peak, callback/rendered/non-silent frames, and output device:
• Browser verification where browser preview remains supported: