Feat/amm registry - #335

Introduce apps/amm/src/RegistryLoader, a QObject that owns the known-tokens /
known-pools snapshot and serves it to the backend's QtRO slots. The JSON
parsers move here; AmmUiBackend now reads tokenList/poolList/resolveTokens from
the loader's snapshot instead of re-parsing the config files inline.

Add the refresh plumbing the UI will drive:
- PROP(int registryRevision READONLY) — bumped on every snapshot refresh so
  QML replicas re-fetch the lists.
- SLOT(void refreshRegistry()) — manual re-load.

No behavior change: the source is still the local TOKENS_CONFIG / AMM_POOLS_CONFIG
files. This is Phase 2a of docs/amm-registry-plan.md; Phase 2b reshapes
RegistryLoader::refresh() into the async remote (AMM_REGISTRY_URL) fetch —
manifest + deployment guard + disk cache — with the same snapshot contract.

When no local TOKENS_CONFIG / AMM_POOLS_CONFIG is set, RegistryLoader now loads
the known-tokens / known-pools from a remote GitHub registry named by
AMM_REGISTRY_URL: an async QNetworkAccessManager fetch of a registry.json
manifest, then the tokens.json / pools.json it points at (resolved relative to
the manifest URL).

- Stale-while-revalidate: the on-disk cache is served immediately and
  revalidated against the network (the manifest `timestamp` is the freshness
  key); a failed/offline fetch keeps the last cache.
- Deployment guard: a manifest whose programIds don't match the app's deployment
  (from configAccount) is rejected; a manifest that omits programIds is trusted.
- Precedence unchanged: local files replace the remote source when set; user
  custom tokens still merge on top.
- registryRevision bumps on each snapshot update; SwapPage/PoolsPage/
  LiquidityPage re-fetch on it. CMake links Qt6::Network.

Add apps/amm/registry-sample/ — a manifest plus empty tokens.json / pools.json
and a README — to exercise the remote path end to end (start with the empty
case). Compile-verified via `nix build .#amm-ui`; not yet run end to end.

Replace the per-network manifest + separate tokens.json/pools.json with a single
Uniswap-token-list-style document: { networks:[{id,name,programIds}],
tokens:[{network,...}], pools:[{network,...}] }. RegistryLoader now fetches one
file, selects the active network, and filters entries to it.

- Active network = AMM_NETWORK (override) -> else the network whose programIds
  match the connected deployment (configAccount) -> else the lone network. The
  programIds match doubles as the deployment guard.
- Registry lives in-repo at artifacts/amm-registry.json (no separate repo yet);
  remove apps/amm/registry-sample/.
- Rename setExpectedProgramIds -> setConnectedProgramIds (drives selection + guard);
  add activeNetwork(). Local dev source (bare arrays) is unchanged.

setup-amm-testnet.sh now also writes tests/testnet/amm-registry.json — the seeded
tokens/pools in the single-file multi-network registry shape (one "local" network
with empty programIds so the lone-network rule auto-selects it; holding-agnostic
tokens; a fresh timestamp each run). This lets the AMM_REGISTRY_URL path be
exercised against the local sequencer without hosting anything — point
AMM_REGISTRY_URL at the file with a file:// URL.