fix(manifest): correct serde feature configuration for no-std

[EliteCoder18]

Description

This PR fixes a bug where the optional serde dependency implicitly leaks the standard library into no-std builds.

Previously, serde was missing default-features = false in the [dependencies] block. When downstream crates attempted to compile miniscript for bare-metal targets (e.g., thumbv7m-none-eabi) using minimal-version dependency resolution, Cargo would resolve serde with its default "std" feature enabled, causing immediate compilation failures.

Notes to the reviewers

I encountered this feature leakage while working on integrating cargo-rbmt CI checks over in bdk_wallet (specifically in this PR: bitcoindevkit/bdk_wallet#494).

When the rbmt matrix runs its minimal dependency sweep on no-std bare-metal targets, it exposes this missing feature configuration. Applying this fix locally allowed the embedded target compilation to pass cleanly.

Changelog notice

• Added default-features = false to the optional serde dependency.
• Explicitly opted into the alloc feature for serde.
• Updated the std feature array to use the weak dependency activation syntax ("serde?/std")

[trevarj]

Nice find. ACK c7adcfd

[apoelstra]

Thanks. Agreed, great fix. In future please do not post so many words. I think this bug was self-evident from the fix itself.

[apoelstra]

ACK c7adcfd; successfully ran local tests