feat(cli): serve the RFC 0006 payment surface from a development purse

[tommyldev]

Summary

truapi-host dev now serves the RFC 0006 payment surface from a local development purse: payment.subscribeBalance and payment.topUp work against a JSON-backed balance instead of failing with PermissionDenied / Unknown("Payments are not supported in dot.li"). To get there, truapi-platform gains a PaymentPurse trait that any host can install on the signing-host runtime, and the CLI installs a dev implementation of it.

Before this, a product could only exercise its balance or top-up flow on the iOS/Android hosts.

What changes

• truapi-platform: new PaymentPurse trait with subscribe_balance(purse) -> BoxStream<Balance> (current value first, then every change) and top_up(purse, amount, source). It is not a Platform supertrait. Like PermissionStatusHost, it is installed set-once on the runtime, so existing platform implementations do not change.
• truapi-server: RuntimeServices holds the installed purse, set through SigningHostRuntime::set_payment_purse. Payment::balance_subscribe and top_up route to it when present. Without a purse the responses are identical to before; wire_result_shape passes unchanged, so desktop and web behaviour does not move. request and status_subscribe stay unimplemented.
• truapi-host-cli: dev_purse.rs keeps per-purse balances in <session>/dev-purse.json. A tokio::sync::watch fan-out gives every subscriber the current balance, then deduplicated changes. top_up credits from a ProductAccount source and rejects PrivateKey and Coins with InvalidSource rather than pretending to spend a pasted secret. Balance disclosure needs no prompt because the CLI already auto-approves every confirmation.
• truapi-codegen: golden host-callbacks.ts gains the additive PaymentPurse interface.

Scope

A top-up credits the purse and moves nothing on chain; the log line says so. The purse is a plain number for exercising product flows on testnets. A wallet-backed host would implement the same trait against real funds.

Verified

• cargo fmt --check, cargo clippy --all-targets on the three crates: clean.
• cargo test -p truapi-host-cli dev_purse (2), -p truapi-server --test wire_result_shape (13, unchanged), -p truapi-codegen (goldens), -p truapi-platform: pass.
• End to end against the Humanity product under truapi-host dev --network previewnet: payment.subscribeBalance pushes 0, payment.topUp(1_250_000, ProductAccount{Index(1)}) returns ok, the subscription pushes 1250000, and dev-purse.json persists it across a restart.

Open questions for review

• Should the CLI gate the dev purse behind a flag (e.g. --dev-purse) rather than installing it unconditionally in dev?
• Should PaymentPurse also cover request / status_subscribe now, or wait for a real backend?

[github-actions]

CI Status: not green, 1 item needs attention.

• rust: failure

All job results

job	result
android-bindings	success
changes	success
codegen	success
e2e	skipped
explorer	success
ios-bindings	success
ios-swift	success
licenses	success
playground	success
rust	failure
ts-client	success
ts-debugger	success
ts-host	success
wasm-provider	success

_{Commit 5709b249 · run log}