event-mapper: decode CAP-67 muxed transfer values and full-range i128 amounts

[AquiGorka]

Root cause (with decoded on-chain evidence)

The empty volume / asset-breakdown tiles were investigated against live testnet data. Findings:

The plain-i128 path works. The two real channel money-flow events on testnet decode correctly with the current code:

• deposit 0016589233871425536-0000000000 (ledger 3862482): transfer GDKSTZ...OBEF -> channel CD5MZC...KPF2, value scvI128 XDR AAAACgAAAAAAAAAAAAAAADuiayA= = 1000500000 stroops
• settlement 0016589324065746944-0000000000 (ledger 3862503): channel -> GBCDUL...QXU7, value scvI128 XDR AAAACgAAAAAAAAAAAAAAAAvrwgA= = 200000000 stroops

Both amounts appear intact in the live WS snapshot's recent payloads. The tiles are empty right now because no deposit/settlement has occurred inside the 24h / 60-min windows since 2026-07-29 13:25 UTC, not because these two events lost their amounts.

Two real decode defects existed regardless:

1. CAP-67 muxed transfer shape drops the amount. Since protocol 23, SAC transfer events whose destination carries a muxed id put their data in an scvMap {"amount": i128, "to_muxed_id": bytes} instead of a bare i128. Real example on testnet: event 0016589229576450048-0000000000 (ledger 3862481). decodeI128 only accepted scvI128, so any muxed deposit/settlement mapped to amount: null and could never reach volume or asset-breakdown aggregates.
2. i128 >= 2^64 decoded to an unparseable string. decodeI128 returned "<hi>:<lo>" whenever hi != 0; the store's parseAmount rejects that, silently dropping large amounts (and all negative amounts, since hi = -1 for those).

Fix

• decodeTransferAmount accepts both on-chain shapes: bare scvI128, and the CAP-67 map (extracts the amount entry).
• decodeI128 does real signed reassembly (hi * 2^64 + lo) and returns a plain decimal string across the full i128 range.

Tests

Five new vectors, the on-chain ones using the verbatim ScVal XDR captured from testnet RPC:

• real deposit event -> channel_deposit with amount 1000500000
• real settlement event -> channel_settlement with amount 200000000
• real CAP-67 muxed map value -> amount decoded (pre-fix: null)
• hi=1, lo=1 -> 18446744073709551617 (pre-fix: "1:1")
• end-to-end replay: real deposit event through mapChainEvent -> recordEvent -> sparkline volume bucket 100.05 and 24h asset breakdown row 1000500000 stroops / 100%

deno task test: 51 passed, 0 failed. check, lint, fmt:check clean.

Live verification (2026-07-31)

Ran local-dev testnet suite 1 through the events-capture harness against the deployed testnet platforms: 5/5 network WS events matched, including channel_deposit amount 100500000 and channel_settlement amount 40000000. Post-run snapshot shows volume sparkline 14.05 XLM, 24h asset breakdown XLM 140500000 stroops (100%), and live latency samples. The plain-i128 path is confirmed live end-to-end; the muxed-map and >=2^64 paths this PR adds are covered by the unit vectors and can get the same live check after deploy.