Add taproot privacy optimisations in policy compiler

[simongltr]

Describe the enhancement

bdk-cli compile --type tr "$POL" currently only calls compile and manually wraps the result into a single taproot leaf, instead of using compile_tr from rust-miniscript that splits disjunctions into separate leaves and Huffman-optimises the tree. Using compile_tr would be a good compromise, though compile_tr_private_experimental exists and goes one step further in privacy optimisation.

Use case

When compiling a disjunctive policy to a taproot descriptor, each spending condition should ideally be its own leaf so that only the used path is revealed on-chain. Currently all conditions are bundled into a single leaf, missing the privacy benefits of taproot's script tree.

Additional context

The following shows the output of all three rust-miniscript functions for reference:

Example 1: or(pk(A),pk(B))

compile:
  or_b(pk(A),s:pk(B))
compile_tr:
  internal key : B
  tap tree leaves (1 total):
    depth=0 script=pk(A)
compile_tr_private_experimental:
  internal key : B
  tap tree leaves (1 total):
    depth=0 script=pk(A)

Example 2: thresh(1,pk(A),pk(B),pk(C))

compile:
  multi_a(1,A,B,C)
compile_tr:
  internal key : C
  tap tree leaves (2 total):
    depth=1 script=pk(B)
    depth=1 script=pk(A)
compile_tr_private_experimental:
  internal key : C
  tap tree leaves (2 total):
    depth=1 script=pk(B)
    depth=1 script=pk(A)

Example 3: thresh(2,pk(A),pk(B),pk(C))

compile:
  multi_a(2,A,B,C)
compile_tr:
  internal key : UNSPENDABLE
  tap tree leaves (1 total):
    depth=0 script=multi_a(2,A,B,C)
compile_tr_private_experimental:
  internal key : UNSPENDABLE
  tap tree leaves (3 total):
    depth=1 script=and_v(v:pk(A),pk(B))
    depth=2 script=and_v(v:pk(B),pk(C))
    depth=2 script=and_v(v:pk(A),pk(C))

[simongltr]

Seems like it's not that simple.

Both compile_tr and compile_tr_private_experimental fail for pk(A):

• compile_tr returns Err("Empty Miniscript compilation")
• compile_tr_private_experimental panics with an assertion failure

Expected result in both cases is tr(A), extract_key correctly promotes A to the taproot
internal key, but the remaining Policy::Unsatisfiable is not handled the same as Policy::Trivial,
causing it to fall through to leaf compilation with nothing to compile.

compile_tr_private_experimental also panics for and(pk(A),pk(B)) (compile_tr is fine there).
Expected tr(UNSPEND,and_v(v:pk(A),pk(B))).