ENG-317: price wire float→double (Phase A, server)

[islandbitcoin]

ENG-317 — float→double on the price wire (Phase A, server side)

Pairs with lnflash/flash#333. Server should land first.

What changes

• realtime/src/servers/protos/price.proto: add double price_v2 = 3
  alongside float price = 1 [deprecated = true] on PriceResponse.
• history/src/servers/protos/price_history.proto: same pattern on
  Tick (float price = 2 [deprecated = true]; double price_v2 = 3;).
• realtime/src/servers/realtime/run.ts: getPrice callback now
  returns { price, price_v2: price } so both fields carry the same
  source value. The float wire encoding still quantises price;
  price_v2 carries the full float64.
• history/src/servers/history/run.ts: listPrices maps each tick to
  { ...t, price_v2: t.price } for the same reason.

Why dual-field instead of in-place float→double

Protobuf wire types for float and double are not interchangeable
(types 5 vs 1) — swapping the type in place would silently corrupt
every reader. The additive deprecate-then-remove pattern is the only
safe rollout. Phase B (follow-up PR after one full release cycle in
prod) drops float price entirely from both protos and stops
populating it on the server.

Acceptance

• Server populates both price and price_v2 from the same source
• No change to upstream price-source code or domain layer
• Wire-compatible with pre-ENG-317 clients (they keep reading
  price and ignore the unknown price_v2 tag)
• Client PR (ENG-317: price wire float→double (Phase A, client) flash#333) prefers price_v2

[linear]

ENG-317 Currency Precision 1.1: Promote `price.proto` float → double

[islandbitcoin]

Paused → converted to draft (not closed)

Re-evaluated against the currency-precision initiative state in Linear (ENG-315/317/319) and the move to IBEX ETH-USDT wallets.

This is the orphaned server half of ENG-317's rollout: the paired client PR (lnflash/flash#333) was closed as a draft back in April, the epic (ENG-315) never started, and every downstream Phase-1 consumer (ENG-319 multi-source aggregator, Phase-4 quote/treasury) is in Backlog. The epic's premise — dust through a coarse USD-cent intermediate — is undercut now that balances are USDT-denominated (micro-unit precision).

ENG-317 moved to Backlog; this PR converted to draft rather than closed, because the change is small, clean, and still mergeable if the re-scope confirms the price-wire float32 is a real precision issue for some pair post-USDT.

To revive: confirm float→double is still needed under USDT wallets, rebase to trigger CI (the repo has workflows but none ran on this branch), and re-open the client side. Context + decision criteria are on ENG-315.