Merge bitcoindevkit/bdk#2421: ci: only build the libraries on MSRV

7bd52322f8b1322c3f11ec8a604401ca21de5f73 ci: only build the libraries on MSRV (志宇)

Pull request description:

  ### Description

  The `MSRV` CI job built all targets and ran the test suite on 1.85.0. Our MSRV promise only covers building the published library crates, and testing on 1.85.0 meant pinning test-only dependencies in `ci/pin-msrv.sh` (currently `proptest`).

  This PR:

  - makes the `MSRV` job build only the libraries (`cargo build --workspace --lib`, `bdk_testenv` included since it declares `rust-version = "1.85.0"`), with no default features and with all features, and drops the test step. Tests already run on `rust-version` in the per-crate `build-*` jobs.
  - removes the `proptest` pin from `ci/pin-msrv.sh`. The `idna`/`icu_*`/`yoke-derive` pins stay, since they come in through `esplora-client`'s `reqwest` -> `url`, which is a library dependency.
  - notes in the README that the MSRV covers the libraries only, not tests or examples.

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  🤖 Generated with [Claude Code](https://claude.com/claude-code)

ACKs for top commit:
  noahjoeris:
    ACK 7bd52322f8b1322c3f11ec8a604401ca21de5f73

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