Treat PSBT version as a minimum and auto-bump for CSV

[ANAMASGARD]

Closes #59

Description

This PR fixes #59 by treating the transaction version in PsbtParams as a minimum version instead of an exact version.

Previously, transaction creation could fail when a selected input or anti-fee-sniping strategy required transaction version 2, but the caller requested version 1.

Now, the transaction version is automatically raised when required.

This PR:

• Renames PsbtParams::version to PsbtParams::min_version.
• Preserves the caller’s requested version when no feature requires a higher version.
• Raises the transaction version to Version::TWO when a selected input requires CSV.
• Raises the transaction version to Version::TWO when anti-fee-sniping selects the nSequence path.
• Preserves caller-provided versions higher than version 2.
• Removes AntiFeeSnipingError::UnsupportedVersion.
• Updates the documentation and changelog.
• Adds regression tests for the new behavior.

Notes to the reviewers

min_version is treated only as a lower bound.

For example:

• min_version: Version::ONE with no CSV input remains version 1.
• min_version: Version::ONE with a CSV input becomes version 2.
• A version higher than 2 is not downgraded.
• The anti-fee-sniping nLockTime path does not unnecessarily raise the version.
• The anti-fee-sniping nSequence path raises the version to 2 when required.

The AFS sequence-path regression test uses a deterministic RNG so it does not depend on random branch selection.

Changelog notice

• Rename PsbtParams::version to PsbtParams::min_version.
• Automatically raise the transaction version for CSV inputs and the AFS nSequence path.
• Remove the unsupported-version AFS error path.

Verification

The following checks passed:

cargo fmt --all -- --check
cargo test --all-features
cargo clippy --all-targets --all-features -- -D warnings
cargo check
cargo test --doc --all-features
RUSTDOCFLAGS="-D warnings" cargo doc --all-features --no-deps
git diff --check

cargo check --no-default-features currently fails inside miniscript because neither its std nor no-std feature is enabled. This appears to be an existing dependency-feature configuration issue and is unrelated to this change.

Before submitting

• I followed the contribution guidelines.
• This PR breaks the existing API.