Mermaid emoji labels render following text as tofu on macOS

Reproduction

On macOS, a Mermaid label containing emoji plus normal text renders the emoji, but the following text becomes tofu boxes:

flowchart LR
    A["🧠 Memory"] --> B["🖥️ Your Mac apps"]
    B --> C["👨‍👩‍👧‍👦 Family + 👍🏽 ready"]

ASCII-only labels render normally.

Environment: macOS aarch64, reproduced on Jcode v0.61.1 using the usvg/resvg 0.46 PNG path.

Diagnosis

Two font-selection behaviors interact:

1. The Mermaid theme starts with Inter and CSS system aliases, followed by platform emoji fonts. The existing helper skips generic aliases. On a stock Mac without Inter, Apple Color Emoji can therefore become the selected "normal" face.
2. During mixed-span shaping, Apple Color Emoji's box glyphs count as present. usvg fallback can then replace a valid normal-font result for the complete text chunk, producing readable emoji followed by tofu.

This is related to the explicit mixed-font run guidance in linebender/resvg#916, with an additional Apple Color Emoji fallback interaction.

Expected behavior

• Emoji use Apple Color Emoji on macOS.
• Normal text uses Inter when installed, otherwise Helvetica through system-ui/sans-serif.
• CJK, math, and other Unicode retain normal font fallback.

Tested patch

I prepared and validated a patch here:

• Commit: https://github.com/brenden7158/jcode/commit/714259da93c098699cdc962a02e73d155d0cba23
• Compare: https://github.com/1jehuang/jcode/compare/master...brenden7158:fix/mermaid-emoji-font-fallback

The patch:

• resolves CSS generic families through fontdb
• maps macOS generic sans-serif to Helvetica when available
• splits generated Mermaid text into grapheme-safe sibling normal/emoji <tspan> runs
• preserves positioning, attributes, and XML entities
• suppresses fallback only for emoji characters already handled by explicit runs
• retains ordinary behavior when no known platform emoji font is installed

Regression coverage includes ASCII, non-emoji Unicode, ZWJ families, skin tones, keycaps, flags, XML entities, positioned/existing tspans, missing emoji fonts, CJK/math fallback, and Helvetica resolution.

Validation performed:

• cargo fmt --all -- --check
• scripts/dev_cargo.sh test -p jcode-tui-mermaid (74 passed)
• scripts/dev_cargo.sh check -p jcode-tui-mermaid --no-default-features
• scripts/dev_cargo.sh clippy -p jcode-tui-mermaid --lib --all-features -- -D warnings
• scripts/dev_cargo.sh build --profile selfdev -p jcode --bin jcode
• local server smoke test and manual macOS render with Apple Color Emoji + Inter + 中文 ∑ fallback

GitHub rejected PR creation from my current CLI authentication context, so I am sharing the ready-to-review commit and compare link here while the corresponding PR is submitted manually.

Start by reviewing the referenced commit and the Mermaid rendering code exercised by jcode-tui-mermaid. Run scripts/dev_cargo.sh test -p jcode-tui-mermaid first, then inspect the listed regression cases and macOS rendering behavior. Done means emoji uses the platform emoji font, normal text remains readable, and the reported validation commands pass.