bdk_chain: widen supported rusqlite version range instead of pinning 0.31.0 #2436

Describe the enhancement

Widen bdk_chain's optional rusqlite dependency from the "0.31.0" pin to a supported version range, e.g. >=0.31.0, <0.41.0, keeping 0.31.x supported while allowing the resolver to select a single newer version when the dependency graph requires it.

Use case

Because libsqlite3-sys sets links = "sqlite3", Cargo rejects any dependency graph containing two incompatible rusqlite versions (see rusqlite#1727). The 0.31.0 pin therefore forces every project that combines BDK with another sqlite consumer onto rusqlite 0.31.x, which is starting to block real upgrades downstream:

• CDK would like to update arti-client (Tor) past 0.22, but arti ≥ 0.23 requires rusqlite ≥ 0.32 (arti 0.47 requires ≥ 0.36, < 0.41), which cannot coexist with the pin. (Previously attempted here in #2002 for the same reason.)
• breez-sdk-spark hit the same wall needing rusqlite 0.32 (#2156).

I understand from #1949 that a straight version bump was deliberately avoided because several projects (BDK/CDK/LDK Node) standardized on 0.31 to keep graphs unifiable. A range addresses that concern directly: nothing forces anyone off 0.31 — graphs that need 0.31 keep resolving to 0.31 — while graphs containing a newer consumer can resolve to one shared newer version.

This is exactly what the other two ecosystem projects did:

• ldk-node: lightningdevkit/ldk-node#1187 — widened to >=0.31.0, <0.40.0, with CI testing across the range.
• cdk-sqlite: cashubtc/cdk#2748 — same range, plus a compatibility fixture running the persistence test suite against every minor release from 0.31.0 to 0.39.0 on MSRV.

Impact

• Blocking production usage
• Nice-to-have / UX improvement
• Developer experience / maintainability

Are you using BDK in a production project?

• Yes
• No
• Not yet, but planning to

Which backend(s) are relevant (if any)?

• Electrum
• Esplora
• Bitcoin Core RPC
• None / not backend-related (e.g. bdk_chain, bdk_core)
• Other (please specify): ____

Project or organization (optional)

CDK (Cashu Development Kit) — cdk-bdk uses bdk_wallet's rusqlite-backed persistence.