ci: individual package tests

[ValuedMammal]

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

cc #1944

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.69%. Comparing base (e095266) to head (ace451e).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2209   +/-   ##
=======================================
  Coverage   77.69%   77.69%           
=======================================
  Files          29       29           
  Lines        5801     5801           
  Branches      271      271           
=======================================
  Hits         4507     4507           
  Misses       1223     1223           
  Partials       71       71           

Flag	Coverage Δ
rust	77.69% <ø> (ø)

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