v0.23.0 — the multi-symbol gateway

The reference gateway catches up with the core. cmd/obgw was the last thing
standing between the multi-symbol design and a venue anyone can run, and converting
it found the bug that refactor was most likely to hide: buildOrder validated the
incoming symbol and then stamped the configured one onto the order, so at a
two-book venue every order silently landed on the first book. A refactor that
compiles is not a refactor that works.

Added

• cmd/obgw serves a set of instruments. Config.Symbols and Config.DataDir;
  one book per symbol — matching goroutine, command log, market-data feed, rate gate
  — under a venue-wide account layer: one Registry, one publisher, one stream per
  account. A client holds one session and sees one ordered conversation whatever mix
  it trades, which is what makes a client id enough to name an order without also
  naming a symbol.

  A one-book venue is unchanged: same config fields, same WALPath and
  SnapshotPath, same small dense ids, and the entire existing test suite passes
  untouched.

• Mass cancel and Query fan across every book and aggregate — an account's orders
  are its orders. Readiness takes the worst book, since a venue with one wedged
  matcher is not ready however healthy the others are.

Fixed

• Routing a cancel needed the session to remember, not the registry to be asked.
  wire.Cancel names an order by ClOrdID alone, so the gateway must pick a book
  without being told which. Resolving the engine order id up front to read its shard
  field is the obvious move and it is wrong: the naming index is written by the
  matching goroutine, so a cancel arriving while its own Enter is still queued
  resolves to nothing and is refused for an order that is about to exist — the
  orphaned-order defect SOAK.md measured at 12,843 orders in thirty
  seconds. The session already knows: it read the Enter, and the Enter carried the
  symbol.

Known limits

• Price gauges do not aggregate across books and report the first one. A last
  trade price averaged over two instruments is not a number; the right answer is one
  series per symbol, which needs label support pkg/observability does not have.

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Upgrading: nothing to do for a one-book venue — Config.Symbol, WALPath and SnapshotPath behave exactly as before, and ids are unchanged. For more than one instrument, set Config.Symbols and Config.DataDir.

Full changelog: v0.22.0...v0.23.0