Merge bitcoindevkit/bdk#2209: ci: individual package tests

Merge bitcoindevkit/bdk#2209: ci: individual package tests

2185dcbb31d512d708e8c117c5b1a429ff52ce81 ci: Update cont_integration.yml (valued mammal)

Pull request description:

  Close #1944

  ### Description

  Replaces the monolithic `build-test` matrix job (which ran a 2-toolchain × 2-feature-set matrix across the whole workspace) with dedicated per-crate build jobs. Each crate uses named steps for every meaningful feature combination, making it easy to identify which specific crate and feature set broke in CI.

  **Changes:**

  - **Per-crate jobs**: `build-core`, `build-chain`, `build-bitcoind-rpc`, `build-electrum`, `build-esplora`, `build-file-store`, `build-testenv` — each with per-feature `cargo build` steps and a `cargo test --all-features` step
  - **`msrv` job**: Simplified from a matrix to a single flat job targeting 1.85.0; runs `cargo build --workspace --all-targets` and `cargo test --workspace --lib --tests --all-features`
  - **`check-no-std`**: Updated to check `bdk_core` and `bdk_chain` with no default features + with `hashbrown` and `miniscript`; cross-compilation against `thumbv6m-none-eabi` is stubbed out as usual to be addressed in a separate PR

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