plan: add Plan::tap_sighash_default accessor

[evanlinjin]

Summary

Adds a public accessor on Plan that exposes the existing per-Schnorr-placeholder size assumption — recorded when the plan was built from TaprootCanSign::sighash_default — as a single Option<bool>.

impl Plan {
    /// Returns whether the witness was sized assuming `SIGHASH_DEFAULT` (the
    /// 64-byte Taproot signature encoding).
    pub fn tap_sighash_default(&self) -> Option<bool>;
}

• Some(true) — every Schnorr placeholder is sized for a 64-byte signature (the SIGHASH_DEFAULT encoding).
• Some(false) — every Schnorr placeholder is sized for a 65-byte signature (any non-Default Taproot sighash flag).
• None — non-Taproot plans (signature size is invariant under the sighash flag for ECDSA), or mixed plans whose Schnorr placeholders have differing sizes — these cannot be expressed by a single uniform sighash policy per BIP-174.

Motivation

Callers building a PSBT from a Plan need to populate PSBT_IN_SIGHASH_TYPE consistently with the plan's witness-size assumption — otherwise weight estimates drift from reality. The information already lives inside the plan (on every Schnorr placeholder's size field); this just surfaces it as a single readable answer.

Concretely: bdk-tx is reworking per-input sighash handling and needs to derive the PSBT sighash policy from the plan rather than carrying separate state on its Input type (see bitcoindevkit/bdk-tx#69 for the full design). Without this accessor it would have to walk Plan::witness_template() and infer the bool from placeholder sizes, which leaks internals.

Test plan

• cargo test --lib tap_sighash_default — new test covers Default-Taproot, non-Default-Taproot, and non-Taproot (wsh) plans.
• cargo test — full suite still passes (139 / 0 / 0).

🤖 Generated with Claude Code

[evanlinjin]

Fully AI generated - I need to ensure it didn't screw up before I take it out of draft.

[evanlinjin]

Closing — wrong target branch. Superseded by #951 (targets master) and #950 (12.x backport). Sorry for the noise.