12.x: plan: add Plan::tap_sighash_default accessor

[evanlinjin]

Summary

12.x backport of #951.

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>;
}

See #951 for full rationale and the test.

Test plan

• cargo build — clean. (No test added on this backport; covered on plan: add Plan::tap_sighash_default accessor #951.)

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