chore: drop unused imports and bindings in the crypto crates

[blacks1ne]

The same mechanical class as #598 but in bls48581, channel, ferret,
vdf, verenc and the four wasm wrappers — 46 warnings — plus five
non_fmt_panics that are real message bugs.

ferret and vdf are edition 2018, where a single-argument panic! takes the
string verbatim. So panic!("unsupported target {target}") prints the literal
{target} rather than the value — the panic message that is supposed to tell you
which target failed does not.

Worth flagging: cargo fix's suggestion for this lint is
panic!("{}", "unsupported target {target}"), which silences the warning and
keeps the broken output. These are fixed with explicit format arguments
instead, so the messages actually interpolate.

Two changes since this was first pushed

• channel-wasm is now in scope (+2). Its two dead use items sit in a
  #[cfg(test)] module and the audit command excluded the crate, so they were
  never listed. With the rest of the series applied they were the last two
  warnings in the workspace.
• One warning moved to chore: annotate the intentionally-unused code paths and drop orphaned test fixtures #602 (−1). json_to_metadata's unused
  ratchet_state parameter is renamed there instead of here, so that the whole
  of that dead function — annotation, comment and parameter — is described by a
  single PR. Previously the two PRs touched adjacent lines of the same signature
  and conflicted on whichever merged second; they now merge in either order and
  produce an identical tree. The warning is still fixed exactly once across the
  series. metadata_to_json, which is live, keeps its rename here.

---

Series

Part of the warning-cleanup series that starts with #597. The eight PRs are
disjoint and each stands on its own, but they are meant to be read in order —
please take #597 first: it is the only one of the eight that fixes a bug
rather than a warning, and it is the shortest.

• Base: 932045a3
• Verified with all eight applied: the workspace goes from 309 warnings to 16 —
  the 16 being classgroup's GMP FFI glue, deliberately left visible rather than
  silenced. Update: those 16 are now fixed rather than documented, in fix: correct classgroup GMP FFI declarations and uninitialized values #605,
  which corrects the declarations and the uninitialized values instead of
  annotating them. With fix: correct classgroup GMP FFI declarations and uninitialized values #605 and the channelwasm commit in this PR, the combined tree
  checks with zero warnings.
• To check this PR on its own:

  cargo check --keep-going --workspace --lib --bins --tests --examples
  

  (The earlier form of this command carried --exclude channelwasm. That crate
  is in scope now — it is the last commit here — so the exclusion is gone.)

Happy to re-pace these, drop any of them, or squash the set into a single PR if
you would rather review it in one pass — just say which.

Drafted with Claude Code; every site was read individually and the reasoning is
in the commit message.

[blacks1ne]

@dazthecorgi You're right in TG comment, and the _ prefix doesn't even remove the work — let _x = expensive() still evaluates.

The line you linked is a real leftover, though not a live bug: decrypt_header
is a copy of the block in advance_and_decrypt minus the self., and the
caller performs the field assignment itself when decrypt_header returns
should_advance_dkg_ratchet = true. So adding the missing self. would write
the field twice rather than fix anything. It is now deleted, with a comment
saying where the real update happens.

Same treatment for the other compute-and-discard binding here: let _M = BIG::new_ints(&rom::CURVE_ORDER) in recurse_fft, allocated on every
recursion level and never read. The eight for (_i, x) in ….enumerate() loops
drop the .enumerate() instead, and the two tuple patterns bind _ rather
than a named placeholder — zip bounds the iteration length there, so the
operands have to stay. ecp.rs's _mask is the one I've kept: that file is
MIRACL Core under its own copyright header.

Pushed as d3dc2f8. I've applied the same review to the rest of the series and
will follow up on the other PRs individually.