feat: RFC 7807 problem-details taxonomy with stable error codes (#267)

[Alfredzee]

• Add core/errors/problem-details.ts with ProblemDetails type, 39-code PROBLEM_CODES registry (validation, Stellar account/asset/trustline/ transaction, Soroban, Horizon transport, RPC transport, internal), getProblemDefinition(), isProblemRetryable(), problemDetailsFromCode(), horizonToProblemDetails(), rpcToProblemDetails()

• Add core/rpc/errors.ts with RpcErrorClassification, classifyRpcError() (maps JSON-RPC standard codes to stable ProblemCode), and classifyRpcTransportError() (maps network/abort/timeout errors)

• Add core/errors/tests/problem-details.contract.test.ts with 36 contract drift tests covering shape validation, registry stability, exhaustive code count, bridge adapters, and safe-message invariants

• Add docs/PROBLEM_DETAILS.md with wire shape, code tables, usage examples, edge cases, failure modes, and rollback/recovery steps

Stability contract: codes are permanent (never rename/remove, only add). Safe-message invariants: no user input or raw transport errors are forwarded to the ProblemDetails envelope.

Summary

Describe what changed and why. One pull request must implement exactly one
issue and one independent feature slice.

Scope proof

Issue: Closes #

Feature directory: features/<slug>/

Changed file count:

git diff --name-only origin/main...HEAD | wc -l

• This pull request changes at least 20 meaningful files (the scaffold normally creates 23)
• Every product change is inside the feature directory above
• This work does not require another open issue or unmerged pull request
• No generated registry file or shared navigation/route list was edited

How to Test

Describe the exact happy path, one validation failure, one transport failure,
and the network(s) a reviewer should use.

1. Go to ...
2. Enter ...
3. See ...

Evidence

Add screenshots for idle, loading, success and error states. Include the result
of the slice-specific contract check:

npm run verify:features -- <slug>

Checklist

• npm run check passes locally
• Logic returns Result<T, Code> for every expected failure
• Every error code has actionable copy in copy.ts
• Idle, loading, success and error states are implemented
• Network requests are tested through MSW, not internal-module mocks
• Accessibility tests cover at least two states with zero WCAG 2.1 A/AA violations
• Amount arithmetic uses strings/BigInt, never floating point
• No secret key is accepted, rendered, stored, logged or transmitted
• README.md records the feature's non-obvious technical decision
• If this changes docs/FEATURE_CONTRACT.md, it adds or supersedes an ADR in docs/adr/

Follow-Up

Call out known limitations only. Do not make this pull request depend on future
work; anything required by the acceptance criteria belongs in this slice.

closes #227

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