feat: Phase 1 — embedded MVP (plan 02)

[sujitn]

Phase 1 embedded MVP per plan 02, with all pin findings recorded in the plan.

• Rust: config/query/writer/catalog natives, full exception mapping with codes coverage test, blocking-collect materialized queries, deep-copy Java→Rust imports
• Java: full io.laminardb API (execute/query/stream/insert/Writer/checkpoint/catalog), ArrowBatch lazy imports, friendly-row conversion
• Gates: SpotBugs + JaCoCo zero-coverage wired into just review
• Tests: 31-test matrix incl QuickstartIT (docs-as-test) and allocator-zero @afterall
• Docs: quickstart, stateful-and-joins, threading, errors, build, benchmarks
• Plan 04 wiring: bundled-natives NativeLoader, release workflow (publish blocked on maintainer credentials — recorded), 4-platform CI

Note: Maven Central publish is a recorded blocker requiring maintainer-only setup (Central Portal namespace, GPG, environment secrets).