feat: handle terminal Paykit payment requests

[dzdidi]

Stack and prerequisites

This is a stacked draft PR. Its base is Locks #61 (feat/lock-payment-draining), which must land first.

Upstream contract order:

1. Paykit Server Bump dtolnay/rust-toolchain from 1.89.0 to 1.100.0 #3
2. Paykit Server fix(connect): match the embedded shell to the app modal #27, stacked on Bump dtolnay/rust-toolchain from 1.89.0 to 1.100.0 #3
3. This Locks rejection-lifecycle change, stacked on Locks feat/lock-payment-draining db reset #61

Development currently uses provisional exact Paykit Server PR #27 commit 44ed37886122a201b2d5f73c9578ecabb48f72bd. Final immutable Paykit Server release tag/pin remains a pre-merge gate. This branch is not release- or deployment-ready while that provisional source ref remains.

Frontend contract

Rejected payment requests surface as status=expired with terminal_reason=payment_request_rejected. They issue no entitlement. Retry requires a fresh Bundle ID.

SDK consumers must migrate terminal handling from status-only branching to the closed terminal_reason values. Unknown, missing, malformed, or conflicting terminal tuples fail closed. Connection-state observation remains separate from payment lifecycle polling.

Summary

• map terminal Paykit rejection/cancellation/expiry outcomes into closed Locks terminal reasons
• persist terminal reason and entitlement-publication intent safely
• fail closed on malformed, unknown, conflicting, or oversized Paykit responses
• expose terminal reason through Rust and browser SDK paths
• provide executable Rust and JS/WASM consumer examples covering lifecycle polling, terminal-reason handling, credential retrieval, proxy reads, and separate connection observation
• document and contract-test provisional stacked demo wiring

SDK consumer tests and docs

• Rust public API tests exercise pending-to-terminal lifecycle lookup, exact terminal reasons, credential/resource-call absence on terminal outcomes, and separate connection observation.
• JS/WASM generated-package smoke tests run copyable consumer examples for accepted, rejected, canceled, and proposal-expired outcomes.
• Consumer docs enumerate all four terminal wire values and require fail-closed migration from status-only handling.

Verification

Exact commit and tree were independently reviewed before publication:

• commit: 35ad885428157bf88c52105ba9aec93290ec5a4c
• tree: 3832e3a802a5af068317fd3e27658e5ad58c4ce3
• exact Locks feat/lock-payment-draining db reset #61 base/head: 7e65742332ed31183c1e23b5680f3c8b8dde10fd

Internal review passed Rust SDK flow tests, JS native/WASM/generated-package tests, example smoke coverage, npm release audit and pack inspection, formatting/diff checks, plus prior workspace, Clippy, rustdoc, browser, and Compose contract checks for the rejection feature. The provisional Paykit Server source build still has the documented upstream-only Criterion::paykit_payment_params compatibility blocker; this is a release/pre-merge dependency, not a regression introduced by these SDK tests/docs and not a claim of runtime readiness.

Prerequisites: #61, pubky/paykit-server#3, pubky/paykit-server#27.