perf(agent): symbol index tak pegang mutex global saat rebuild I/O

Sebelumnya SemanticSearch/ListSymbols/RebuildIndex menahan SYMBOL_INDEX
Mutex selama full `rebuild` (walk seluruh workspace, bisa detikan) +
selama search. Di main loop yang menjalankan read-only tools paralel,
semantic_search/list_symbols lain jadi BLOCK selama rebuild.

Refactor:
- Global berubah Mutex<Option<SymbolIndex>> -> OnceLock<Mutex<HashMap<
  workspace, SymbolIndex>>> — index per-workspace, jadi pencarian workspace B
  tidak mungkin bocor simbol stale dari A (workspace-awareness kini struktural,
  bukan hanya via needs_rebuild).
- ensure_symbol_index(workspace, force): rebuild dijalankan DI LUAR lock
  (mutex hanya dicek/insert/lookup singkat), lalu hasilnya di-swap-in di bawah
  short lock. Search/list/rebuild-report tak lagi memblock thread lain selama
  walk I/O. Per-workspace key menghilangkan race lintas-workspace dari skema
  swap tunggal.
- Test +1 (test_ensure_symbol_index_per_workspace_isolation): verifikasi dua
  workspace punya index independen, rebuild A tidak menimpa B.

Verifikasi: check/clippy -D warnings/fmt clean; test infra 64 (0 gagal).

Cargo.lock

@@ -4862,7 +4862,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-api"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "argon2",
@@ -4885,7 +4885,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-application"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "base64",
@@ -4903,7 +4903,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-bootstrap"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "chrono",
@@ -4920,7 +4920,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-daemon"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "base64",
@@ -4944,7 +4944,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-domain"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "base64",
@@ -4960,7 +4960,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-gateway"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "axum",
@@ -4987,7 +4987,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-grpc"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "axum",
@@ -5004,7 +5004,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-infrastructure"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "argon2",
@@ -5052,7 +5052,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-tui"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "base64",
@@ -5078,7 +5078,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-web"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "axum",
@@ -5098,7 +5098,7 @@ dependencies = [
 
 [[package]]
 name = "zesdex-ws"
-version = "1.20.2"
+version = "1.21.0"
 dependencies = [
  "anyhow",
  "axum",