docs: add SDK package boundary and dependency direction map

[jd5073356-max]

Summary

docs/dependency_direction_map.md existed only as a 4-line stub — a title and one sentence referencing "core, utils, and contract clients," none of which match any real module in this SDK. This replaces it with a real map, verified directly against src/'s actual imports rather than inferred from module names or copied from architecture.md's existing (partially stale) diagram.

Per the acceptance criteria in #285:

• Architecture boundary map added: full per-module ownership/dependency table covering all 12 real src/ directories. architecture.md's existing layer diagram only documented 9 of them — account, errors, and vault exist in src/ with real, substantial implementations and were undocumented anywhere in the repo.
• Module responsibilities explained: added the three missing module sections to architecture.md in its existing format, plus the full ownership table in the new map.
• Dependency direction documented: account is correctly placed in the infrastructure layer (it's an identity/signing abstraction with a pluggable Signer + hardware/mobile-signer extension point), not a peer feature module — even though only transactions has adopted it so far. vault is documented as a pure re-export facade over soroban with no implementation of its own. And the config↔diagnostics mutual reference — which looks exactly like the cycle the layer rule forbids — is explained precisely: diagnostics/hooks.ts (a leaf config depends on) and diagnostics/report.ts (which depends on config) never import each other, so npm run check:circular reports zero cycles despite the module-level mutual reference. Worth knowing before adding a new import between the two, since the circular-dependency checker operates on individual files, not directories.
• Security-sensitive modules highlighted: account/wallet are the only modules that ever touch secret key material, errors owns redaction, diagnostics has its own independent redaction layer.
• Cross-module integration examples included: correct vs. incorrect examples for config resolution, deep imports, and adopting the account abstraction.
• README links added: neither new doc was linked from README.md's Documentation section before this PR — architecture.md (already large and useful) was effectively undiscoverable.

Pre-existing, out-of-scope finding

While verifying the repo before touching docs, npm run verify reported 46 pre-existing test failures across 5 files (retry-policy.test.ts, types/asset.test.ts, and others) — functional bugs in retry classification and asset validation, unrelated to module structure. npm run lint (tsc --noEmit) and npm run check:circular both pass cleanly, which is what this doc's claims actually depend on, so these failures don't block anything in this PR. Flagging for visibility rather than expanding this PR's scope into unrelated bug fixes.

Test plan

• npm run lint (tsc --noEmit) — passes
• npm run check:circular — passes, 46 modules, 0 cycles
• Every dependency edge in the new map was verified with grep -rhoE "from '\.\./[a-z]+" src/<module> per module, not assumed
• Every relative link in the new/edited docs resolves to a real file
• README links to both architecture.md and the new dependency map

Closes #285