chore(vestige-core): drop async-trait dependency

cargo rm async-trait. Last usage was the FastembedEmbedder impl attribute,
removed in the preceding 0001c commit; the MemoryStore side stopped using
async-trait at 0001a.

Verification: grep -rn async_trait crates/ returns zero hits. grep -rn
async-trait --include=Cargo.toml crates/ returns zero hits. Cargo.lock no
longer references the async-trait package.

Cargo.lock

@@ -164,17 +164,6 @@ dependencies = [
  "stable_deref_trait",
 ]
 
-[[package]]
-name = "async-trait"
-version = "0.1.89"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn",
-]
-
 [[package]]
 name = "atomic-waker"
 version = "1.1.2"
@@ -4686,7 +4675,6 @@ checksum = "0b928f33d975fc6ad9f86c8f283853ad26bdd5b10b7f1542aa2fa15e2289105a"
 name = "vestige-core"
 version = "2.1.26"
 dependencies = [
- "async-trait",
  "blake3",
  "candle-core",
  "chrono",

crates/vestige-core/Cargo.toml

@@ -127,7 +127,6 @@ usearch = { version = "=2.23.0", optional = true }
 lru = "0.16"
 trait-variant = "0.1"
 blake3 = "1"
-async-trait = "0.1"
 
 [dev-dependencies]
 tempfile = "3"