feat(payments): add payment request schema and parser

[emirykl]

Summary

• add versioned payment request types, strict Zod validation, JSON/object parsing, expiry enforcement, and typed non-echoing errors
• integrate parsed requests into the web payment intent builder and register valid, expired, malformed, and unsupported fixtures
• document the v1 format, validation rules, parser API, UI workflow, and trust boundaries

Test evidence

• pnpm --filter @anchorkit/stellar-kit test -- paymentRequest.test.ts — 10 tests passed
• TypeScript package suites — 330 tests passed; web integration suite — 2 tests passed using an explicit in-repository Vitest config
• relevant workspace type checks passed
• focused package lint and full web lint passed
• pnpm check:examples — 14 examples passed
• pnpm check:boundaries passed
• stellar-kit build and Next.js production build passed
• focused Prettier check, git diff --check, and secret-pattern scan passed

Known baseline issues: repository-wide lint still reports unrelated findings in validationEngine.ts, assetRegistry.ts, and receipt.ts; root format:check attempts to parse Rust without a configured parser and flags unrelated baseline files. Default web Vitest discovery searches above the repository, so its in-repository test was rerun successfully with an explicit safe config.

Fixes #84

Verification

• PASS: payment request parser tests (10)
• PASS: TypeScript package suites (330 tests)
• PASS: web integration tests (2 with explicit in-repository Vitest config)
• PASS: relevant workspace type checks
• PASS: focused package lint and full web lint
• PASS: example consistency check (14 fixtures)
• PASS: package boundary check
• PASS: stellar-kit and Next.js production builds
• PASS: focused formatting, git diff --check, and secret-pattern scan
• Baseline: broad lint has unrelated existing findings; root format:check is broken by missing Rust parser support and unrelated formatting

Diagnostics

Recovered command failures: /bin/zsh -lc "find . -name AGENTS.md -print && rg -n \"parse.*Result|ParseError|safeParse|Date\\(|expires|UNSUPPORTED|PaymentRequest\" packages apps docs tests examples scripts --glob '"'!**/dist/**'"' --glob '"'!**/.next/**'"'" (2), /bin/zsh -lc "test -d node_modules && echo 'node_modules present' || echo 'node_modules missing'; node --version; pnpm --version; pnpm --filter @anchorkit/stellar-kit test -- paymentRequest.test.ts" (127), /bin/zsh -lc 'corepack pnpm --filter @anchorkit/stellar-kit test -- paymentRequest.test.ts' (1), /bin/zsh -lc 'corepack pnpm exec turbo run build --filter=@anchorkit/stellar-kit...' (1), /bin/zsh -lc 'corepack pnpm --filter @anchorkit/anchor-utils build && corepack pnpm --filter @anchorkit/types --filter @anchorkit/config --filter @anchorkit/validators --filter @anchorkit/stellar-kit --filter @anchorkit/anchor-utils --filter @anchorkit/web test' (1), /bin/zsh -lc 'corepack pnpm --filter @anchorkit/types --filter @anchorkit/config --filter @anchorkit/validators --filter @anchorkit/stellar-kit --filter @anchorkit/anchor-utils --filter @anchorkit/web lint' (1), /bin/zsh -lc 'corepack pnpm format:check && git diff --check' (2)

[El-swaggerito]

\nThis PR is currently blocked by merge conflicts.\n\nPlease update the branch with the latest main branch and resolve the conflicts before it can be merged.

[El-swaggerito]

\nGitHub has not finished calculating whether this PR can be merged cleanly.\n\nThe auto-merge automation will skip this PR for now. Re-run the automation later.