feat: introduce TxTemplate as an intermediate stage

[evanlinjin]

Summary

Closes #57. Refactors Selection into a richer tx-shape type called TxTemplate. The pipeline becomes:

Selector::try_finalize() → TxTemplate → (Psbt, Finalizer) | Transaction

TxTemplate is the single workspace for how the transaction is shaped — version, locktime, fallback sequence, per-input sequence overrides, ordering, anti-fee-sniping, and final emission. AFS is no longer privileged: it's a pure caller of public methods (set_locktime, Input::set_sequence).

New templates start with sensible defaults (version = v2, lock_time = max input CLTV (or ZERO), fallback_sequence = ENABLE_RBF_NO_LOCKTIME). All overrides are methods: set_version, set_locktime, set_fallback_sequence, apply_anti_fee_sniping, shuffle_inputs, sort_outputs_by, input_mut, etc.

Migration

Before	After
Selection	TxTemplate
Selector::try_finalize() -> Option<Selection>	Selector::try_finalize() -> Option<TxTemplate>
InputCandidates::into_selection(algo, params)	InputCandidates::into_tx_template(algo, params) -> Result<TxTemplate, _>
IntoSelectionError	IntoTxTemplateError
PsbtParams	PsbtBuildParams (emission-only)
PsbtParams::version	TxTemplate::set_version(v) -> Result<_> (rejects v < 2 if any input has a relative timelock)
PsbtParams::min_locktime	TxTemplate::set_locktime(lt) -> Result<_> (validates same-unit + ≥-CLTV)
PsbtParams::anti_fee_sniping: Option<Height>	TxTemplate::apply_anti_fee_sniping(tip, &mut rng) -> Result<_>
selection.create_psbt(p)?	tx_template.create_psbt(PsbtBuildParams::default())? returning (Psbt, Finalizer)
selection.shuffle_inputs(&mut rng) (&mut self)	tx_template.shuffle_inputs(&mut rng) (consuming, returns Self)
selection.sort_inputs_by(...)	tx_template.sort_inputs_by(...)
selection.input_mut(op) / inputs_mut()	tx_template.input_mut(op) / inputs_mut()
(hardcoded ENABLE_RBF_NO_LOCKTIME)	TxTemplate::set_fallback_sequence(s); default unchanged

Behaviour changes worth flagging:

• What used to be silent is now an explicit error. min_locktime of the wrong unit was silently ignored; below-CLTV was silently clamped up. Now set_locktime returns SetLockTimeError::UnitMismatch / SetLockTimeError::BelowInputCltv. Trying to set v1 with a relative-timelock input returns SetVersionError::RelativeTimelockRequiresV2.
• The hardcoded Sequence::ENABLE_RBF_NO_LOCKTIME fallback is now configurable via set_fallback_sequence.
• Library no longer depends on rand. Only rand_core (for the RngCore trait). Production dep tree: miniscript, bdk_coin_select, rand_core.

Out of scope

• no_std feature. Blocked on bdk_coin_select upstream: fix: add conditional FloatExt import for no_std builds coin-select#37 (FloatExt import fix) is merged but no 0.4.2 release exists yet. Follow-up once a release lands.
• PSBT v2 support. Design accommodates it cleanly as a future TxTemplate::create_psbt_v2() method — same construction logic, different emit step.

Notes for reviewers

• apply_anti_fee_sniping calls set_locktime / Input::set_sequence via the public path and .expect()s the results. The expects hold by construction:
  • locktime path: only fires when afs_locktime >= current (both height-based, since AFS early-rejects time-based), so set_locktime's unit + ≥-CLTV checks always pass.
  • sequence path: only picks taproot inputs without a relative timelock, so set_sequence cannot fail the CSV/CLTV-disable check.
• TxTemplate::from_parts (the constructor used by Selector::try_finalize) is pub(crate). External users construct via Selector / InputCandidates. If hand-rolled TxTemplate becomes a common ask, exposing this is a one-line change.

Commits (3)

1. `refactor!: rename Selection to TxTemplate` — pure rename, no behaviour change. Sets up commit 2.
2. `feat(tx-template)!: route tx-shape decisions through TxTemplate` — the meat. Adds the resolved tx-shape fields (`version`, `lock_time`, `fallback_sequence`), the corresponding setters with validation (`set_version`, `set_locktime`, `set_fallback_sequence`), and the refactored PSBT/AFS pipeline (`PsbtParams` → `PsbtBuildParams`, `create_psbt` returns `(Psbt, Finalizer)`, AFS becomes a chainable `apply_anti_fee_sniping` step calling public methods). Closes Introduce TxTemplate as a state between Selection and Psbt #57.
3. `refactor!: drop rand dependency from the library` — `create_psbt` no longer needs an RNG; `rand` moves to dev-deps. Production tree is now `miniscript`, `bdk_coin_select`, `rand_core`.

Test plan

• CI: `cargo fmt --check`, `cargo clippy --all-features`, `cargo check --all-features --tests --examples`, `cargo test --all-features`
• Doc tests pass (the synopsis example in `finalizer.rs` was updated to the new API)
• Manual run of `examples/synopsis` and `examples/anti_fee_sniping` against bitcoind via `bdk_testenv`
• Verify no transitive `rand` dep in `cargo tree` against the published library

[evanlinjin]

This is fully AI generated and not ready for review.