fix: lock sync's first inspection, resolve symlinks without their target

Fourth Codex (gpt-6.1-sol, xhigh) pass over 5e0831d, each item verified
before fixing:

- sync inspects the local database under the shared lock, so another
  sync cannot swap it under that connection.
- symlinks are resolved by reading the link (db::resolve_db_path), not
  canonicalize, so a symlinked path finds the same lock and install
  target even while the real file is missing (fresh install, mid-swap)
  or the link dangles.
- readers go ahead unlocked when no lock file can be created; nothing
  can replace the database there. (A read-only data directory still
  fails in SQLite's WAL setup, as in 0.2.4.)
- XLSX shared strings are capped at 2,000,000 entries.

Verified live: an install waits for a reader's shared lock; a symlinked
db_path waits on the real path's lock and stays a symlink. CLI goldens
unchanged; live A/B vs 0.2.4: 16 passed, discover hits the IDX 403 on
both builds (exit 4 vs 1, intended).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/idx-core/src/ownership/xlsx.rs

@@ -127,7 +127,10 @@ fn parse_shared_strings(xml: &str) -> Result<Vec<String>, IdxError> {
                 b"rPh" => in_phonetic = true,
                 _ => {}
             },
-            Ok(Event::Empty(e)) if e.local_name().as_ref() == b"si" => strings.push(String::new()),
+            Ok(Event::Empty(e)) if e.local_name().as_ref() == b"si" => {
+                check_shared_string_count(strings.len())?;
+                strings.push(String::new())
+            }
             Ok(Event::Text(text)) if in_text && !in_phonetic => {
                 if let Some(value) = current.as_mut() {
                     let unescaped = text
@@ -137,7 +140,10 @@ fn parse_shared_strings(xml: &str) -> Result<Vec<String>, IdxError> {
                 }
             }
             Ok(Event::End(e)) => match e.local_name().as_ref() {
-                b"si" => strings.push(current.take().unwrap_or_default()),
+                b"si" => {
+                    check_shared_string_count(strings.len())?;
+                    strings.push(current.take().unwrap_or_default())
+                }
                 b"t" => in_text = false,
                 b"rPh" => in_phonetic = false,
                 _ => {}
@@ -184,6 +190,18 @@ fn text_cap_error() -> IdxError {
     )
 }
 
+const MAX_SHARED_STRINGS: usize = 2_000_000;
+
+fn check_shared_string_count(count: usize) -> Result<(), IdxError> {
+    if count >= MAX_SHARED_STRINGS {
+        return Err(xml_error(
+            "shared strings",
+            format!("more than {MAX_SHARED_STRINGS} entries"),
+        ));
+    }
+    Ok(())
+}
+
 fn check_row_count(rows: usize) -> Result<(), IdxError> {
     if rows >= MAX_SHEET_ROWS {
         return Err(xml_error(