[Bug] markdown: lone ~ treated as strikethrough, breaks range/math notation (e.g. 350~500ml)

[Badegg404]

Problem

parseGfm / parseGfmWithMath (packages/client/ui-primitives/src/markdown/parse.ts) use micromark-extension-gfm, which defaults singleTilde: true.

In Chinese and technical prose, ~ is commonly used as a range or approximate sign (e.g. 350~500ml, 1~1.5 升). With singleTilde enabled, a lone ~ pairs with a later ~ and the text in between renders as strikethrough.

Repro:

parseGfm('350~500ml 的小瓶，那才 1~1.5 升，更没问题。')

The span ~500ml 的小瓶，那才 1~ is struck through. GitHub itself ignores lone ~ — only ~~double-tilde~~ is strikethrough per GFM spec.

Root cause

micromark-extension-gfm's singleTilde option defaults to true, allowing a single ~ as a strikethrough delimiter.

Fix

Pass gfm({ singleTilde: false }) so strikethrough strictly requires ~~ and lone ~ renders literally. Applies to both parseGfm and parseGfmWithMath, covering both rendering arms (streaming and settled).

Tests

Added 2 regression cases to markdown-dom-parity.client.spec.tsx with matching fixtures:

• single-tilde-range-not-strikethrough — a range like 350~500ml is no longer struck through
• single-tilde-escaped-and-bare — escaped tilde, real ~~strikethrough~~, and bare single tildes all behave correctly

pnpm vitest run packages/client/ui-primitives/tests/markdown-dom-parity.client.spec.tsx — 52 passed.