refactor(hashline-edit): stabilize hashes and tighten prefix stripping

Switch line hashing to significance-aware seeding so meaningful lines stay stable across reflows while punctuation-only lines still disambiguate by line index.

Also narrow prefix stripping to hashline/diff patterns that reduce accidental content corruption during edit normalization.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/hashline-edit/hash-computation.ts

@@ -1,10 +1,12 @@
 import { HASHLINE_DICT } from "./constants"
 import { createHashlineChunkFormatter } from "./hashline-chunk-formatter"
 
+const RE_SIGNIFICANT = /[\p{L}\p{N}]/u
+
 export function computeLineHash(lineNumber: number, content: string): string {
-  const stripped = content.replace(/\s+/g, "")
-  const hashInput = `${lineNumber}:${stripped}`
-  const hash = Bun.hash.xxHash32(hashInput)
+  const stripped = content.endsWith("\r") ? content.slice(0, -1).replace(/\s+/g, "") : content.replace(/\s+/g, "")
+  const seed = RE_SIGNIFICANT.test(stripped) ? 0 : lineNumber
+  const hash = Bun.hash.xxHash32(stripped, seed)
   const index = hash % 256
   return HASHLINE_DICT[index]
 }