chore: trim the Midnight comments down to what is worth reading

[ctoyan]

Comment-only pass over the Midnight work, in two rounds: the first rewrote the
overlong comments, the second deleted the ones that should not have existed at
all. Net ~900 lines of comment removed.

What went

• Docs that only echoed the item name. toolkit.rs alone had about a
  hundred: the same /// Operation discriminator / /// Indexer GraphQL endpoint (HTTP) block repeated across six near-identical request structs.
  Nothing in that module is reachable outside the crate, so none of it was
  required by missing_docs — confirmed by clippy after removal.
• Module and crate headers written as essays. The Rust siblings
  (hyperlane-cosmos, hyperlane-aleo, hyperlane-sealevel) all open with a
  single line, so these do too, keeping only what is not visible from the code:
  the feature gate, the off-chain merkle tree, the Bytes<N> trailing-zero
  trim. merkle_tree_hook.rs went 36 header lines to 10, metadata_tests.rs
  31 to 12, state_decode.rs 22 to 7.
• Comments restating the assertion beneath them. A line reading "a shared
  prefix does not match" sitting above an assert that a shared prefix does not
  match tells nobody anything.
• Cross-chain trivia. Whether a choice matches EVM, Aleo, or Sealevel is
  interesting while writing the code and useless while reading it. It stays
  only where it grounds a whole design (the off-chain merkle tree).
• Every #N issue reference, including two in runtime strings: a todo!()
  message and an error message a user could actually see.
• Match-arm narration. The Midnight arms in chains.rs, cursors/mod.rs
  and chain.rs each had a comment describing the line below it, where every
  surrounding arm has none. One was also stale — it still described reading
  count/current_root from an on-chain merkle tree that was removed. The
  state_decode.rs header was stale too: it listed field positions ([0, 8],
  [0, 9]) that contradicted the actual constants directly below it.
• Version detail that dates fast (compact-js, wallet-sdk, midnight-js, node)
  where the behaviour is the point. The reason the chunked deploy is hand-built
  at the ledger level is still there; the exact version that forced it is not.
• Stale claims. The midnight-sdk README no longer says "under
  construction", and the PARTIAL_SUCCESS note no longer says segment
  attribution is still being verified — that was confirmed safe.
• core-config.yaml trimmed from a 14-line preamble to 5, matching the
  other example configs.

Three changes that are not comments

Flagging these since everything else is comment-only:

1. Deleted announce_and_read_back_on_devnet in validator_announce.rs. It was
   #[ignore] plus unimplemented!() and a four-step plan in a comment — it
   never ran and never asserted anything. Say the word if you'd rather keep the
   placeholder.
2. Renamed tx_status_filter_is_the_single_decision_point to
   tx_status_filter_excludes_only_failures, so the name says what it asserts.
3. Dropped an unnecessary mut in mailbox.rs.

Verification

• cargo test -p hyperlane-midnight — 84 passed, 3 ignored
• cargo clippy -p hyperlane-midnight --all-targets — no missing_docs, no new
  warnings (one pre-existing is_multiple_of suggestion, untouched by this PR)
• cargo check -p hyperlane-base -p relayer -p lander -p validator --features midnight — clean
• cargo fmt --all --check — clean
• tsc --noEmit on sdk and cli; builds green for midnight-sdk, provider-sdk,
  deploy-sdk
• oxlint clean on every touched TS file
• Diffed every changed file with comment lines stripped, against both the base
  and the first round: the only non-comment changes are the three above.