query: peg-based query language for ad-hoc graph exploration

poc-memory query "degree > 15"
poc-memory query "key ~ 'journal.*' AND degree > 10"
poc-memory query "neighbors('identity.md') WHERE strength > 0.5"
poc-memory query "community_id = community('identity.md')" --fields degree,category

Grammar-driven: the peg definition IS the language spec. Supports
boolean logic (AND/OR/NOT), numeric and string comparison, regex
match (~), graph traversal (neighbors() with WHERE), and function
calls (community(), degree()). Output flags: --fields, --sort,
--limit, --count.

New dependency: peg 0.8 (~68KB, 2 tiny deps).

Cargo.toml

@@ -14,6 +14,8 @@ libc = "0.2"
 faer = "0.24.0"
 rkyv = { version = "0.7", features = ["validation", "std"] }
 memmap2 = "0.9"
+rayon = "1"
+peg = "0.8"
 
 [build-dependencies]
 capnpc = "0.20"