fix: resolve 5 deployment blockers (runtime-fallback race, hashline legacy, tmux spawn, db open)

- runtime-fallback: guard session.error with sessionRetryInFlight to prevent
  double-advance during active retry; expand session.stop abort to include
  sessionAwaitingFallbackResult; remove premature pendingFallbackModel clearing
  from auto-retry finally block
- hashline-edit: add HASHLINE_LEGACY_REF_PATTERN for backward-compatible
  LINE:HEX dual-parse in parseLineRef and normalizeLineRef
- tmux-subagent: defer session on null queryWindowState; unconditionally
  re-queue deferred session on spawn failure (not just close+spawn)
- ultrawork-db: wrap new Database(dbPath) in try/catch to handle corrupted DB
- event: add try/catch guards around model-fallback logic in message.updated,
  session.status, and session.error handlers

src/tools/hashline-edit/validation.test.ts

@@ -52,3 +52,46 @@ describe("validateLineRef", () => {
     expect(() => validateLineRef(lines, "1#ZZ")).toThrow(/current hash/)
   })
 })
+
+describe("legacy LINE:HEX backward compatibility", () => {
+  it("parses legacy LINE:HEX ref", () => {
+    //#given
+    const ref = "42:ab"
+
+    //#when
+    const result = parseLineRef(ref)
+
+    //#then
+    expect(result).toEqual({ line: 42, hash: "ab" })
+  })
+
+  it("parses legacy LINE:HEX ref with uppercase hex", () => {
+    //#given
+    const ref = "10:FF"
+
+    //#when
+    const result = parseLineRef(ref)
+
+    //#then
+    expect(result).toEqual({ line: 10, hash: "FF" })
+  })
+
+  it("legacy ref fails validation with hash mismatch, not parse error", () => {
+    //#given
+    const lines = ["function hello() {"]
+
+    //#when / #then
+    expect(() => validateLineRef(lines, "1:ab")).toThrow(/Hash mismatch|current hash/)
+  })
+
+  it("extracts legacy ref from content with markers", () => {
+    //#given
+    const ref = ">>> 42:ab|const x = 1"
+
+    //#when
+    const result = parseLineRef(ref)
+
+    //#then
+    expect(result).toEqual({ line: 42, hash: "ab" })
+  })
+})