feat(explorerextensions): per-alkane tx indexes (txs_by_alkane + internal_txs_by_alkane)

[kungfuflex]

What

Adds a new espo module explorerextensions that maintains two
trace-derived reverse indexes, keyed by alkane id, to power an
etherscan-style account view for an alkane:

• explorerextensions.txs_by_alkane — transactions whose top-level
  cellpack target (depth 0 in the alkanes trace, i.e. the EOA-level
  "to") is the alkane.
• explorerextensions.internal_txs_by_alkane — transactions that
  reach the alkane via an internal call (call / delegatecall /
  staticcall) at any depth > 0 — the alkanes analog of etherscan
  "internal txns". Each row carries the internal call frames
  (call_type + caller).

Params for both: { alkane: "block:tx", page?, limit? } (1-indexed page,
default limit 50, cap 500). Results are newest-first and paginated.

Why

There was no alkane-id-keyed index for transaction history at all. The
essentials.get_alkane_balance_txs* methods cover balance-changing
activity, but not "what txs targeted this alkane" or "what txs touched it
internally" — which is exactly what an explorer/wallet needs to render a
contract page like etherscan's Transactions / Internal Txns tabs.

How

Both indexes are derived purely from the alkanes execution trace already
attached to every EspoBlock transaction, so indexing needs no extra
metashrew calls. index_block walks each trace's event stream with a
depth stack (same shape as extract_alkane_storage): the outermost
Invoke.myself is the top-level target; anything entered at depth > 0 is
an internal call.

• Storage: standard module Mdb (RocksDB), keyed
  <tag><block:4 BE><tx:8 BE><height:4 BE><txid:32> so a prefix scan
  returns an alkane's whole history in order; values are borsh metadata
  (vout/status/opcode for top-level; call frames for internal).
• The explorerextensions: namespace is added to the versioned set in
  mdb.rs, so it inherits the same tree-db reorg rollback as the other
  modules (and falls back to plain RocksDB in unit tests, where the global
  tree-db is unset).
• Registered after essentials in main.rs; no config section required.

Tests

src/modules/explorerextensions/tests.rs — hermetic unit tests
(synthetic traces, temp RocksDB, no metashrew/network) proving:

• top-level vs internal separation (an alkane delegatecalled internally
  shows up in internal_txs_by_alkane, not txs_by_alkane);
• status / opcode / caller decoding;
• idempotent re-indexing (height gate, no duplicate rows).

Verified locally: cargo test --lib explorerextensions → 2 passed.

Note: to run the build locally I had to temporarily pin the alkanes-rs
git deps to the last pre-"v3-align" commit
(b9a8a9f), because as of today alkanes-rs@develop moved the
metashrew-* crates out into kungfuflex/metashrew (they're now in the
workspace exclude list), so metashrew-support = { git = alkanes-rs, branch = "develop" } no longer resolves. That pin is not part of
this PR (deps are left on branch = "develop"); flagging it separately
since it currently blocks espo's build against develop regardless of
this change.