chore(ci): replace pin-msrv.sh with cargo-rbmt and committed lockfiles

[EliteCoder18]

Description

Closes #400
Closes #79
Part of #410

Replaces the fragile ci/pin-msrv.sh dependency-pinning script with a two-lockfile strategy using cargo-rbmt the same tooling used by the rust-bitcoin project. This implements the approach

Problem

ci/pin-msrv.sh pinned specific transitive dependency versions via cargo update --precise at build time. This was brittle: every time a dependency chain changed, someone had to manually discover which packages needed pinning and update the script by hand. It also couldn't be replicated locally without running the same script in the same order.

Solution

Two versioned lockfiles are now committed to the repository:

Cargo-minimal.lock :- Minimal compatible versions used for the MSRV build
Cargo-recent.lock :- Recent/latest compatible versions used for the stable build

CI copies the appropriate lockfile to Cargo.lock before building and passes --locked to cargo build/cargo test, giving fully reproducible builds that contributors can replicate locally.

A new check-lockfiles CI job runs cargo rbmt lock and fails if the committed lockfiles drift from what cargo-rbmt would generate, so staleness is caught automatically on every PR.

Files changed

• Cargo-minimal.lock && Cargo-recent.lock: committed lockfiles generated by cargo rbmt lock
• ci/install-rbmt.sh: installs cargo-rbmt at the version pinned in rbmt-version
• rbmt-version: pins cargo-rbmt at 0.3.0 for auditable, cached installs
• Cargo.toml: adds [package.metadata.rbmt.*] config with toolchain versions and feature exclusions for no-std; fixes bdk_chain (hashbrown) and miniscript (no-std) feature flags for no-std targets; pins dev-dependency versions for lock stability
• .github/workflows/cont_integration.yml: replaces pin-msrv.sh step with lockfile copy and --locked; adds check-lockfiles job
• CONTRIBUTING.md: documents how to regenerate lockfiles when Cargo.toml changes
• ci/pin-msrv.sh: deleted

Notes to the reviewers

• The [package.metadata.rbmt.test].exclude_features list in Cargo.toml tells cargo-rbmt which features to skip in its matrix — needed to avoid pulling std-only crates (anyhow, bdk_file_store, etc.) into no-std feature combos.
• The nightly toolchain entry under [package.metadata.rbmt.toolchains] is required by cargo-rbmt's.
• This mirrors exactly how rust-bitcoin manages their MSRV CI see their Cargo-minimal.lock for reference.
• To regenerate the lockfiles after a Cargo.toml change:

  ./ci/install-rbmt.sh
  cargo rbmt toolchains
  cargo rbmt lock
  

Changelog notice

• chore(deps): add Cargo-minimal.lock and Cargo-recent.lock lockfiles; fix no-std feature flags for bdk_chain (hashbrown) and miniscript (no-std)
• chore(ci): replace ci/pin-msrv.sh with cargo-rbmt; copy lockfile before build and add --locked; add check-lockfiles job to detect stale lockfiles on every PR
• docs: document lockfile regeneration workflow in CONTRIBUTING.md and README.md

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran just p before pushing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.96%. Comparing base (39de6ed) to head (5789c83).
⚠️ Report is 48 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
+ Coverage   80.93%   80.96%   +0.02%     
==========================================
  Files          24       24              
  Lines        5482     5489       +7     
  Branches      247      247              
==========================================
+ Hits         4437     4444       +7     
  Misses        968      968              
  Partials       77       77              

Flag	Coverage Δ
rust	80.96% <ø> (+0.02%)	⬆️

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[EliteCoder18]

The cargo-rbmt lockfile workflow itself is expected to run successfully. However, updating this PR before the upstream fixes are released would cause the no_std CI matrix to fail: serde can still pull std in indirectly through miniscript and bdk_chain/bdk_core.

The relevant fixes are rust-miniscript#987 and bdk#2227. Once both changes are released, bdk_wallet can update to those released versions, regenerate the rbmt lockfiles, and rerun the full CI matrix. After that, this PR should be ready for review and merge.