Sort module attributes when flattening

[cemde]

Bug

return tuple(module.__dict__.values()), tuple(module.__dict__.keys())

BaseModule flattened on __dict__ insertion order, which is the order a constructor happened to assign attributes in.

Impact

Two instances of the same class built by different code paths get different treedefs:

class M(pxc.Module):
    def __init__(self, swap):
        if swap: self.b, self.a = Param(1.0), Param(2.0)
        else:    self.a, self.b = Param(2.0), Param(1.0)

jtu.tree_structure(M(False)) == jtu.tree_structure(M(True))   # False

They cannot be tree_maped together, and passing one then the other to a jitted function forces a recompile. A module is documented as flattening "as if it were a dictionary", and jax's own dict flattening is deliberately key-order-insensitive for exactly this reason.

Fix

Sort the keys in both flatten variants. unflatten_module rebuilds __dict__ by zipping aux keys with children, so it consumes whatever order flatten emits and stays consistent automatically.

Why a test changed

test_extract_yields_one_entry_per_dynamic_param_in_traversal_order asserted that extracted[0] was w, the first-assigned attribute. Its docstring states the contract as "extraction has to be deterministic and skip statics"; which deterministic order is precisely what this PR changes, so the positional literals recorded incidental behaviour rather than the contract. No order normalisation of any kind could preserve them. The two expected values are swapped and the docstring now says why.

Verified

A full two-moons run is bit-identical to main across per-batch energies, test accuracy and every weight elementwise. tree_ref/tree_unref still restores object identity for an aliased param, and a jitted function called with a wb-ordered and a bw-ordered instance now traces once instead of twice. Old checkpoints still load, since keys are jtu.keystr attribute paths rather than positions.

Reordering a float32 reduce can move the last bits of a sum. It does not on the tutorial, whose module attributes are already alphabetical and whose Vodes live in a list, but a constructed worst case shifts the total energy by 5e-6 relative. Inherent to any order normalisation.

Closes #76