feat(convert): add LynxDB backend

[mostafa]

Summary

• Add a new rsigma-convert backend targeting the LynxDB log analytics engine, generating SPL2-compatible FROM <index> | search <predicates> queries
• Handle LynxDB-specific semantics: glob wildcards (*), deferred | where clauses for regex/CIDR/single-char wildcards, CASE() for case-sensitive matching, and explicit AND-group parenthesization for LynxDB's non-standard boolean precedence (NOT > OR > AND)
• Register backend in CLI (-t lynxdb), add unit tests (30+ cases covering all value types, modifiers, boolean logic, deferred expressions, output formats), golden tests for 9 representative Sigma rules, and documentation in both crate and root READMEs

What's included

Backend (crates/rsigma-convert/src/backends/lynxdb/mod.rs)

• LYNXDB_CONFIG (TextQueryConfig) with LynxDB-specific tokens, operators, wildcards, quoting, and expression templates
• LynxDbBackend implementing the Backend trait with overrides for:
  • convert_condition_and / convert_condition_or: filter empty strings from deferred items and parenthesize AND groups
  • convert_field_eq_re / convert_field_eq_cidr: return ConvertResult::Deferred for | where pipeline stages
  • finish_query: apply pipeline state (e.g. custom index) before defaults
  • finalize_query: support default and minimal output formats

Tests

• 30+ in-module unit tests covering equality, wildcards, modifiers (contains/startswith/endswith/cased), regex, CIDR, numeric comparisons, boolean logic, field existence/null, keywords, deferred expressions, output formats, and index override
• 9 golden tests with committed .yml + .expected pairs: simple_eq, and_or_not, wildcards, regex, cidr, keywords, exists_null_bool, numeric_compare, brute_force

CLI & docs

• Registered lynxdb target in get_backend() and cmd_list_targets()
• Updated CLI snapshot tests
• Updated crates/rsigma-convert/README.md with usage examples, output formats, index selection, modifier mapping table, precedence and deferred expression documentation
• Updated root README.md with CLI examples and architecture diagram

Test plan

• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets --all-features -- -D warnings passes
• cargo test --workspace passes (all existing + new tests)
• Golden tests produce expected LynxDB query output
• CI passes on all matrix targets (ubuntu, macOS, windows)

@OrlovEvgeny Please test and provide feedback.