Logo knockout shape/border customization + reconcile ECC-level gating policy

Two related gaps around logo embedding vs. the original plan:

1. **Knockout shape/border**: `LogoOptions` only supports a plain background-colored rectangle knockout behind the logo. The original plan mentioned an "optional knockout bg shape/border" (e.g. rounded rect, circle, or a visible border/outline around the knockout).
2. **ECC-level gating**: the original plan said to "only allow [logos] with Q/H" error correction levels. The current implementation (`LogoLayoutCalculator`) instead allows a logo at *any* ECC level, just with a different max size percent per level (L=15%, M=20%, Q=25%, H=30%). Need to decide whether to keep the more permissive current behavior (with a stronger warning at low ECC) or restrict to Q/H as originally planned, and document the decision.

**Scope:**
- Add optional knockout shape (rect/rounded-rect/circle) and border/outline styling to `LogoOptions`.
- Decide and implement (or explicitly reject) ECC-level gating for logo embedding.
- Add/adjust unit tests accordingly.

Start with LogoOptions and LogoLayoutCalculator, then inspect the existing unit tests for logo embedding. Clarify the supported knockout shapes and border styling, resolve whether ECC gating remains permissive or is restricted to Q/H, and update tests and documentation so the chosen behavior is covered.

Written by the indexing model from the issue text.