fix(chain,core): prevent serde from implicitly leaking std into no_std environments

[EliteCoder18]

Description

Fix silent serde/std leak in bdk_core and bdk_chain both crates declared their serde optional dependency without default features = false, causing serde's std feature to activate unconditionally even in no_std builds.

• bdk_core: added default-features = false + serde?/std in std feature.
• Dropped rc the only two serde-derived types (BlockId, ConfirmationBlockTime) are pure scalars needing neither serde/rc nor serde/alloc.
• bdk_chain: added default-features = false and explicitly enabledalloc and rc directly on the serde dep + serde?/std in std. Both are required because tx_graph::ChangeSet holds BTreeSet<Arc<Transaction>> and serde gates Arc<T>: Serialize behind all(feature = "rc", any(feature = "std", feature = "alloc")).

This fix was identified while working on (bitcoindevkit/bdk_wallet#494).

Notes to the reviewers

The alloc and rc features are placed on the serde dependency directly rather than behind a separate feature flag because bdk_chain unconditionally requires alloc anyway.

Changelog notice

• bdk_core: serde dep was missing default-features = false, leaking serde/std into no_std builds and remove unnecessary rc feature .
• bdk_chain: same leak fixed and serde/alloc and serde/rc now explicitly enabled on the dep .

Checklists

All Submissions:

• I followed the contribution guidelines

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.65%. Comparing base (6d03fc3) to head (29d1f5b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2227   +/-   ##
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  Coverage   78.65%   78.65%           
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  Files          30       30           
  Lines        5909     5909           
  Branches      279      279           
=======================================
  Hits         4648     4648           
  Misses       1185     1185           
  Partials       76       76           

Flag	Coverage Δ
rust	78.65% <ø> (ø)

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[notmandatory]

ACK cc4cef4

Good catch finding that rc isn't needed for bdk_core.

[oleonardolima]

utACK cc4cef4

I merged the update of rustc, so you'll need to rebase it run with the latest CI.

[oleonardolima]

nit: do we need this, as it's already being brought by bdk_core/std and bdk_core/serde below ?

[EliteCoder18]

You're right, removed. Since both crates are in the same repo the transitive path through bdk_core/std is guaranteed. Thanks!