Add FilterBar query language parser (pest grammar)

Introduces a standalone src/filter_parser.rs module (no dependency on
engine/index types) that parses the FilterBar query language into a
FilterExpr AST using a pest grammar (src/filter_parser.pest). Supports
regex("...") searches, metric_name[param] <op> value comparisons,
reserved sequence/timestamp metrics, and and/or combinators with
correct precedence (and binds tighter than or) and parenthesized
sub-expressions. Evaluation against real log data is left to a later
integration ticket.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

Cargo.toml

@@ -10,3 +10,5 @@ tower-http = { version = "0.6", features = ["fs", "cors"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 clap = { version = "4", features = ["derive"] }
+pest = "2"
+pest_derive = "2"