docs(rust): rewrite the README as a crates.io landing page

[lan17]

rust/README.md is what crates.io renders for the dialcache crate, and until now it never said how to install it. This rewrite gives the Rust README the shape of the TypeScript README and makes every fact scannable for an engineer evaluating the crate.

• Open with the use-case definition, the experimental notice, nine capability bullets (all under 80 columns) and one link line that includes docs.rs.
• Add an Install section: cargo add, a feature table, and the MSRV rule (1.85; 1.88 for the redis feature because of the locked redis 1.x dependency).
• Show both entry points in the Usage snippet (use_case().register() and get_or_load) plus get_uncached, followed by five gotcha bullets.
• Replace fact-list paragraphs with tables for Policy leaves and instance defaults; every paragraph is one idea and under 60 words.
• Remove protocol-internal terms (request memo, flight, settlement, quiescence) and second person; name the real builder knob shadow_max_in_flight instead of "shadow capacity".
• Move contributor-only details (settlement control, driver causality, DIALCACHE_RUST_SUITE) out; they live in formal/PORTING.md and formal/WALKTHROUGH.md.

Validation: the Usage snippet was extracted into a temporary example and passed cargo check, rustfmt --check and cargo run (asserts hold; it prints Hello, User 42 <ada@example.com>!). All 17 links and 3 heading anchors resolve. Every factual claim was re-checked against rust/src (defaults in limits.rs, builder methods, Error variants, Policy::validate, feature gates) and the locked redis crate's rust-version.

Scope: README prose only. No code, Quint model, conformance assertion or docs-site page changed. rust/README.md is not part of the source audit, so no ledger refresh is needed. Prose length is unchanged at about 2,070 words; readability comes from structure, not deletion.