docs: use pinned Batter Git dependencies

[featherenvy]

Replace sibling-path installation examples in the README and relocation guide with copyable Git dependencies pinned to merged Batter commit 70cc6a05be6857aca6ea2f5f127258e75e673d8c. Include optional PostgreSQL, HTTP and Axum dependencies and use the same Git pin in the Serde feature example. Explain that related native and Batter packages should share one Git URL and revision.

Follow-up to #10. The selected Batter revision contains both Runlimit and Runledger. No library code, manifests, lockfiles or database migrations change.

Validation: isolated Git consumers built and ran the README memory example on Rust 1.94.0 with all five native packages, both without and with the optional memory Serde feature. Cargo metadata confirms a single Git revision without sibling paths or consumer patches. Dependency-example consistency, Markdown fences and whitespace checks passed. Required contributor checks passed: formatting, strict workspace Clippy, workspace tests, packaged-source consumer checks and native database tests against disposable PostgreSQL 16.15. Executable README examples are unchanged.

Native Codex review of the complete pinned correction diff reported no findings. Documentation validation caught a remaining registry pin in the Serde example; that pin was corrected and its Git configuration verified.

Owning task: runlimit-wwb.

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