[Google Sans Flex] Incomplete ss01 coverage for Vietnamese stacked diacritics (ặ, ầ, ậ)

**Describe the bug**
When stylistic set 1 (`ss01`) for single-story 'a' is enabled, the OpenType substitution (`GSUB`) table in Google Sans Flex fails to cover Vietnamese characters with compound/stacked diacritics.

While basic single-accent characters (like à, á, ả, ã, ạ) correctly render as a single-story 'a', stacked diacritic combinations (such as ặ, ầ, ậ, ẳ, ẵ, etc.) lack single-story variants in the `ss01` lookup list. As a result, the font falls back to the default double-story 'a' for those specific glyphs within Vietnamese text strings.

**To Reproduce**
1. Load Google Sans Flex.
2. Enable stylistic set 1 (`font-feature-settings: "ss01" 1;` or `font-variant-alternates: stylistic(ss01);`).
3. Render Vietnamese text containing double diacritics, such as `Cài đặt` or `Cần thiết`.

**Expected behavior**
All 'a' variants with diacritics in Vietnamese (including compound accents like ặ, ầ, ậ) should render as single-story 'a'.

**Screenshots**
*(Drag and drop your UI screenshot here)*

**Additional context**
Observed when rendering UI strings in Vietnamese where single-accent glyphs properly map to `ss01`, but compound/stacked accents fall back to default double-story glyphs.