Merge #987: fix(manifest): correct serde feature configuration for no-std

c7adcfd fix(manifest): correct serde feature configuration for no-std (Rishit Modi)

Pull request description:

  ### 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"`)


ACKs for top commit:
  trevarj:
    Nice find. ACK c7adcfd
  apoelstra:
    ACK c7adcfd; successfully ran local tests


Tree-SHA512: 33c2993856cfa70cba83f4b0f95b5bd9df04d6a50db3485d13ec18ea4d7c9f613f50ad86e6b53f1ad9cf7642165cd67d736a3e81f8f9b480084bb38a67ad728d

Author: apoelstra

Cargo.toml

@@ -13,7 +13,7 @@ rust-version = "1.63.0"
 
 [features]
 default = ["std"]
-std = ["bitcoin/std", "bitcoin/secp-recovery", "bech32/std"]
+std = ["bitcoin/std", "bitcoin/secp-recovery", "bech32/std", "serde?/std"]
 compiler = []
 trace = []
 
@@ -26,7 +26,7 @@ bech32 = { version = "0.11.0", default-features = false, features = ["alloc"] }
 bitcoin = { version = "0.32.6", default-features = false }
 hex = { package = "hex-conservative", default-features = false, features = ["alloc"], version = "1.0.0" }
 
-serde = { version = "1.0.103", optional = true }
+serde = { version = "1.0.103", default-features = false, features = ["alloc"], optional = true }
 
 [dev-dependencies]
 serde_test = "1.0.147"