fix(editor): stop escaping a lone ~ so Go-template prose round-trips byte-exact

faithfulEscape backslash-escaped every ~, but in GFM only ~~ is strikethrough —
a lone ~ is literal. Escaping it injected a stray \ into prompt prose (e.g.
~10%/~30% in generator.tmpl + refiner.tmpl), and since the editor's getMarkdown
output is the Go text/template stored and sent to the LLM, the \ shipped to the
model. Escape ~ only in runs of 2+ (a balanced ~~strike~~ is handled by the
Strike mark upstream and never reaches this path); backtick/backslash unchanged.

Pinned by byte-identity tests for lone tildes (the corpus test asserts only
word-multiset + convergence, so \~ slipped through).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/components/shared/editor-markdown.ts

@@ -24,8 +24,12 @@ const createFaithfulMarked = () => {
 // ``` ` * _ [ ] ~ ``` — both corrupt our content (tags become entities, `[1-1000]`/`*.php`/`snake_case`
 // gain stray backslashes). Text serialization is hard-coded in the manager (no per-extension hook), so we
 // retune that one method: drop the entity-encoding entirely, and backslash-escape only the chars that
-// would otherwise re-parse as inline syntax (`` ` ``, `~`, `\`).
-const faithfulEscape = (text: string): string => text.replace(/([\\`~])/g, '\\$1');
+// would otherwise re-parse as inline syntax: `` ` `` and `\` always, and `~` only when doubled — a lone
+// `~` is literal in GFM, so escaping it would inject a stray `\` into prose like `~10%`.
+const faithfulEscape = (text: string): string =>
+    text.replace(/[\\`]|~+/g, (match) =>
+        match[0] === '~' ? (match.length > 1 ? match.replace(/~/g, '\\~') : match) : `\\${match}`,
+    );
 
 type ManagerWithEncode = {
     codeTypes: Set<string>;

frontend/src/components/shared/markdown-editor-extensions.test.ts

@@ -63,6 +63,19 @@ describe('selective escape — no stray backslashes on literal punctuation', ()
     );
 });
 
+describe('lone tilde stays literal — no stray backslash injected (M1)', () => {
+    it.each(['~10% for environment setup', 'approximately ~5 minutes', 'a ~ b ~ c'])(
+        'keeps %s byte-identical',
+        (s) => {
+            expect(roundTrip(s)).toBe(s);
+        },
+    );
+
+    it('still round-trips ~~strikethrough~~ (double tilde unaffected)', () => {
+        expect(roundTrip('use ~~deprecated~~ here')).toContain('~~deprecated~~');
+    });
+});
+
 describe('inline marks round-trip', () => {
     it.each(['**bold**', '*italic*', '`code span`', '~~strike~~', '[a link](https://example.com)', '**bold `code` end**'])(
         'preserves %s',