feat(filterquery): add schema-agnostic boolean filter core

[jiashuoz]

Summary

• define the q filter language and its AIP-160-style boolean precedence
• add a schema-agnostic lexer, parser, public AST, validation registry, and parameterized SQL emitter
• enforce parser/traversal caps, fixed identifiers, LIKE escaping, and injection invariants
• add golden conformance vectors, precedence enumeration, fuzz coverage, and manual-AST hardening tests

Why

This establishes a reusable, safe filter-language core before binding it to the messages API. Schema-specific fields and transport integration are intentionally isolated in the follow-up stacked PR.

Validation

• go test ./internal/filterquery/... -count=1
• 30-second parser fuzz run
• independent correctness review: approved
• adversarial security/contract review: approved

Stack

PR 1 of 2. The follow-up branch feat/filter-query-language targets this branch and adds message-store/API/SDK/CLI/MCP integration.