fix(hashline): accept legacy hashes for indented anchors

Keep persisted LINE#ID anchors working after strict whitespace hashing by falling back to the legacy hash for validation-only lookups.

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

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

@@ -1,6 +1,7 @@
 import { describe, it, expect } from "bun:test"
 import {
   computeLineHash,
+  computeLegacyLineHash,
   formatHashLine,
   formatHashLines,
   streamHashLinesFromLines,
@@ -58,6 +59,19 @@ describe("computeLineHash", () => {
     expect(hash1).not.toBe(hash2)
   })
 
+  it("preserves legacy hashes for leading indentation variants", () => {
+    //#given
+    const content1 = "function hello() {"
+    const content2 = "  function hello() {"
+
+    //#when
+    const hash1 = computeLegacyLineHash(1, content1)
+    const hash2 = computeLegacyLineHash(1, content2)
+
+    //#then
+    expect(hash1).toBe(hash2)
+  })
+
   it("ignores trailing whitespace differences", () => {
     //#given
     const content1 = "function hello() {"