docs: reposition README as a standalone KV store; benchmarks, changelog, progress catch-up

[qdequele]

Documentation-only, plus one crate-doc example (verified compiling).

• README repositioned as a pure-Rust, LMDB-design KV store (standalone + heed drop-in), simplified to essentials, with a usage example; cross-engine throughput table (NVMe Graviton public suite) + read-side context; removes the non-existent v0.1.0 tag from the install snippets.
• benches/results/2026-09-30-public-suite-nvme.md (new): full per-engine tables (throughput, latency, RSS, disk) backing the README table.
• CHANGELOG [Unreleased]: the post-0.1.0 perf campaign + ADR-0014/0015/0017/0018 + dependency refresh.
• PROGRESS.md: backfill 2026-09-25 → 10-01 (campaign, cross-engine suite, ADR-0018 amendment merged, ADR-0019 parked, ADR-0020 spike).
• crates/zerodb/src/lib.rs: refresh the stale module doc + add a compiling no_run example.

No engine code changes.

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Reviewing files that changed from the base of the PR and between 2a69c79 and 79ca3d7.

📒 Files selected for processing (5)

• CHANGELOG.md
• PROGRESS.md
• README.md
• benches/results/2026-09-30-public-suite-nvme.md
• crates/zerodb/src/lib.rs

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