feat: implement SDK unsupported feature and capability error standard (#284)

[XxHugheadxX]

Closes #284

Summary

Implements the standard specified in docs/capability_error_standard.md, adding
UnsupportedFeatureError and CapabilityMismatchError and routing the
config-gated vault, Soroban and wallet paths through them.

Root cause

ERROR_CODES[VAULT_CONTRACT_NOT_CONFIGURED].developerHint (src/errors/codes.ts:288)
tells integrators to "Set SDKConfig.contractId before vault calls". But
resolveContractId (src/soroban/index.ts:51) only read params.contractId and
process.env.VAULT_CONTRACT_ID — never the config, even though all three vault
entry points accept a Partial<SDKConfig>.

It also threw the raw string MISSING_CONTRACT_ID, absent from the registry, so
isKnownErrorCode returned false and describeError fell through to
"An unexpected error occurred." fundTestnetAccount had the identical defect with
TESTNET_ONLY. Following the SDK's own documented path produced a generic error.

Changes

• src/errors/unsupported.ts (new) — both error classes, extending
  PocketPayError, carrying module, operation, capability and a
  suggestedNextStep read from the registry's developerHint so guidance cannot
  drift. toJSON() gives a log-safe view.
• src/errors/capabilities.ts (new) — capability registry with four statuses:
  supported, config-gated, planned, unsupported, plus assertCapability().
• src/soroban/index.ts — resolveContractId resolves params →
  SDKConfig.contractId → VAULT_CONTRACT_ID → STELLAR_CONTRACT_ID.
• src/soroban/client-factory.ts — simulation failures use the registry's
  SOROBAN_SIMULATION_FAILED; the vec parameter type reports through
  UnsupportedFeatureError.
• src/wallet/index.ts — fundTestnetAccount uses WALLET_TESTNET_ONLY.
• Docs — expanded capability_error_standard.md; updated configuration.md,
  error-handling.md, soroban-vault.md.

Planned versus supported

The registry classifies real cases rather than aspirational ones:

• signer.local — supported.
• signer.remote — planned. The Signer interface is deliberately async so
  remote signers can replace LocalSigner (src/account/signer.ts:83-86), but
  none ships; consumers may implement Signer themselves.
• soroban.param-type.vec — unsupported. ScValType advertises vec, but the
  underlying Stellar SDK rejects it. Verified against @stellar/stellar-sdk 13.3.0:
  of the 17 declared types exactly one fails (invalid type: vec). A test asserts
  that throw, so this is reclassified if a future SDK adds support.

No status promises a delivery date; a test asserts that no registry or capability
message contains one.

Error codes

SDK_NOT_IMPLEMENTED and VAULT_CONTRACT_NOT_CONFIGURED were declared with full
specs but had zero call sites; they get their first ones here. One code is added,
WALLET_TESTNET_ONLY, for a network-gated capability that had no registry entry —
codes.ts explicitly allows this ("Add new ones; never edit/recycle existing
ones"). Nothing is renamed or recycled.

Backwards compatibility

• params.contractId and VAULT_CONTRACT_ID keep working unchanged.
• Both classes extend PocketPayError, so existing catch blocks and the
  instanceof PocketPayError re-throws in the vault functions are unaffected.
• Breaking: MISSING_CONTRACT_ID → VAULT_CONTRACT_NOT_CONFIGURED and
  TESTNET_ONLY → WALLET_TESTNET_ONLY. Neither old string was in the registry.
  Documented in a Migration section. The vault message still contains "contract ID",
  so mapSorobanContractError() and its existing test are unaffected.

Implementation note

PocketPayError's constructor ends with
Object.setPrototypeOf(this, PocketPayError.prototype), which overwrites the
prototype a subclass gets from new.target. Both subclasses restore their own
prototype after super(); without it, instanceof on the subclass silently
returns false and name stays 'PocketPayError'. Two tests cover this.

Tests

34 new tests in tests/unsupported-feature.test.ts covering both classes, the
capability registry, describeError(err.code).known flipping false to true on the
vault and wallet paths, each of the four contract-ID resolution sources,
per-operation metadata, redactError staying safe, instanceof PocketPayError
preserved, and the no-overpromising assertion. tests/fund.test.ts updated for the
new wallet code.

Verification

lint, check:circular (39 modules, no cycles) and build all pass.

Full suite: 46 failed · 677 passed · 1 skipped. The 46 failures are pre-existing
on the base commit — verified by running the suite against a clean checkout of
d7403d8, which gives 46 failed · 643 passed · 1 skipped: same failures in the
same five files (config-validation, destination-validation, types/asset,
retry-policy, payments). This branch adds exactly the 34 new passing tests and
introduces no regressions.

Scope

Limited to the capability-gated paths the issue names. A wider grep found ~25
further error codes outside the registry in payments, transactions and utils; those
are validation errors rather than capability errors and are left untouched rather
than widening this PR.