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

src/filter_parser.pest

@@ -0,0 +1,42 @@
+// Grammar for the FilterBar query language.
+//
+// Supports:
+//   - regex("...")                      raw-text regex search over log lines
+//   - metric_name[param] <op> value     metric comparisons
+//   - sequence <op> value               reserved metric name (no [param])
+//   - timestamp <op> value              reserved metric name (no [param])
+//   - `and` / `or` boolean combinators, with `and` binding tighter than `or`
+//   - parenthesized sub-expressions
+
+WHITESPACE = _{ " " | "\t" | "\n" | "\r" }
+
+file = { SOI ~ or_expr ~ EOI }
+
+or_expr = { and_expr ~ (or_kw ~ and_expr)* }
+and_expr = { term ~ (and_kw ~ term)* }
+
+term = _{ regex_expr | comparison | "(" ~ or_expr ~ ")" }
+
+// These must be atomic (`@`): a non-atomic rule would let pest's implicit
+// whitespace insertion slip in between the keyword and its own boundary
+// check below, which breaks the lookahead (it would see the first
+// non-whitespace character *after* skipping the separating space, instead
+// of the character immediately following the keyword).
+and_kw = @{ "and" ~ !(ASCII_ALPHANUMERIC | "_") }
+or_kw = @{ "or" ~ !(ASCII_ALPHANUMERIC | "_") }
+
+regex_expr = { "regex" ~ "(" ~ string_literal ~ ")" }
+
+string_literal = ${ "\"" ~ inner_str ~ "\"" }
+inner_str = @{ (escaped_char | !("\"" | "\\") ~ ANY)* }
+escaped_char = @{ "\\" ~ ANY }
+
+comparison = { metric ~ ("[" ~ param ~ "]")? ~ op ~ value }
+
+metric = @{ ident }
+param = @{ ident }
+ident = @{ (ASCII_ALPHANUMERIC | "_")+ }
+
+op = { "<=" | ">=" | "==" | "!=" | "<" | ">" }
+
+value = @{ (!(WHITESPACE | ")") ~ ANY)+ }