fix(render): preserve forced HDR encoder mode

[miguel-heygen]

Summary

• Preserve the resolved forced-HDR mode when selecting the final encoder, even for SDR-only compositions.
• Keep native-HDR resource detection separate so layered native-HDR compositing behavior is unchanged.
• Add an exact regression for forced HLG with zero native HDR resources.

Root cause

The renderer correctly resolved --hdr on an SDR-only composition to HLG and logged BT.2020 10-bit H.265. Later, encoder selection gated that resolved mode on the presence of native HDR resources and passed undefined to the preset resolver. The render therefore exited successfully with SDR H.264/BT.709 despite the HDR diagnostic.

Validation

• Red/green forced-HDR encoder regression.
• Focused HDR mode tests: 8 passed.
• Full producer suite: 296 unit tests plus all classified producer lanes passed.
• Producer typecheck passed.
• Formatting, lint, diff check, and pre-commit checks passed.

Human review is required; this PR is not self-approved or merged.