fix(core,chain): disable serde default features for no_std

[ANAMASGARD]

Description

Fixes #2208.

Both bdk_core and bdk_chain declared their serde dependency without default-features = false. Since serde's default feature set includes std, enabling bdk_chain/serde (or bdk_core/serde) in a downstream no_std crate activated serde/std through Cargo's feature unification, breaking compilation on bare-metal targets where std is unavailable.

This PR disables serde's default features in both crates and re-enables serde/std only through each crate's own std feature.

Changes

• bdk_core / bdk_chain: serde = { version = "1", optional = true, default-features = false, features = ["derive", "rc", "alloc"] }
  • alloc mirrors the bitcoin crate and is required for no_std + alloc serialization
  • rc is kept unchanged (existing behavior)
• Propagate serde/std only via each crate's std feature using serde?/std
• CI (check-no-std job): added a cargo tree feature-graph assertion (--no-dev-dependencies) that fails if serde/std is pulled into a no_std build, plus a thumbv7em-none-eabihf bare-metal cargo check

Notes to the reviewers

cargo check --no-default-features --features serde on the host passes even before this fix, because the host provides std. The bug only surfaces on targets without std, so verification uses cargo tree -e features (dependency-graph proof) and a bare-metal target (compile proof).

Verified locally:

• no_std + serde no longer pulls serde feature "std" for bdk_core and bdk_chain
• std + serde still enables serde/std (no behavior change for std users)
• Full test suite green: bdk_core, bdk_chain, bdk_file_store, bdk_testenv, bdk_bitcoind_rpc, bdk_electrum, bdk_esplora
• cargo clippy --all-features --all-targets -- -D warnings, cargo doc, and cargo +nightly fmt --all --check all pass

Follow-up (out of scope): a no_std + miniscript + serde combination may still pull serde/std via upstream miniscript's serde dependency, which does not disable its default features. That is a separate upstream concern and is not addressed here.

Changelog notice

• Fixed bdk_core and bdk_chain incorrectly enabling serde/std in no_std builds when the serde feature was enabled.

Checklists

All Submissions:

• I followed the contribution guidelines

Bugfixes:

• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR