Move track-assembler fallback speech policy into declarative config

[ctoth]

Problem

track-assembler.ts still owns several speech-policy fallback constants in the frame assembly path. They are not merely plumbing; they change timing and F0 when upstream declarative data is absent.

Evidence

• src/track-assembler.ts has fallback duration logic around fallbackDuration.
• src/track-assembler.ts uses a hard segmentStart + 0.02 floor for transition timing.
• src/track-assembler.ts falls back to baseF0 / 2 when voiced event F0 underflows.

Acceptance criteria

• Move these fallback policies to explicit declarative config with citations or engineering-estimate labels.
• Validate required policy values at load time instead of silently applying inline defaults.
• Add focused tests for missing-policy failure and configured-policy behavior.
• Keep assembly code as orchestration over already-resolved policy, not the owner of fallback values.
• npm.cmd run typecheck:core passes.