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

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