feat: add shared DB utilities for URL params, schema detection, and path expansion

Adds to loader::db: parse_identifier_params, strip_prefer_params,
validate_identifier_name, detect_strategy, expand_dotted_paths,
set_nested_value, and expand_columnar trait method on ConfigLoader.

Supports three auto-detected strategies (kv, raw, wide) with
configurable table/column names, path separator, and row filtering
via URL query parameters.

Author: monokrome

prefer/Cargo.toml

@@ -32,6 +32,9 @@ dirs = { version = "5.0", optional = true }
 # Plugin registration (std only)
 inventory = { version = "0.3", optional = true }
 
+# URL parsing for database identifiers (std only)
+url = { version = "2", optional = true }
+
 # Optional format support (std only, no serde)
 roxmltree = { version = "0.20", optional = true }
 rust-ini = { version = "0.20", optional = true }
@@ -52,6 +55,7 @@ std = [
     "dep:notify",
     "dep:dirs",
     "dep:inventory",
+    "dep:url",
 ]
 json5 = ["std"]  # JSON5 features handled by jzon fallback
 xml = ["std", "dep:roxmltree"]