docs: add end-to-end developer journey walkthrough (Closes #66)

[ALIPHATICHYD]

Summary

Closes #66

Create an end-to-end developer journey document (docs/DEVELOPER_JOURNEY.md) showing how AnchorKit modules work together from account diagnostics to payment readiness, anchor lifecycle, and escrow milestones.

Issue scope

• End-to-end developer journey is documented.
• Account diagnostics flow is included.
• Payment readiness flow is included.
• Anchor lifecycle flow is included.
• Escrow milestone flow is included.
• Examples and tests are linked.

Stellar correctness

• Any new Stellar public-key / secret-key parsing matches the branded types.
• Amounts remain 7-decimal strings (not number / floats) through the change.
• Memo rules are enforced if touched.
• Horizon error handling still maps to the typed StellarKitError codes correctly.

Security impact

• R0: No new console.log / logger / error .message contains a raw secret.
• R1: No secret echoed in UI text, copy-to-clipboard, or URL.
• R2: No new localStorage / cookie / persistent store of secrets.
• R3: No secret in a constructed URL, callback, query param, or auth header.
• R4: Structural validation runs before SDK calls that could leak into stack traces.
• R5: All new secret-accepting APIs accept unknown, Zod-validate, and return branded types.
• R6: No committed real secret keys anywhere in fixtures/examples.

Tests

• New public functions in packages/ have a positive and a negative Vitest case.
• Any new branch in error mapping or status transitions has a test.
• For contract changes: happy path + error path tests added under src/test.rs covering the specific new guard.
• Tests do not print or assert on raw secret keys.

Documentation updates

• /docs/DEVELOPER_JOURNEY.md updated for any new user-facing package export.
• README or /docs/LOCAL_SETUP.md updated if setup steps changed.

No secret leakage + no mainnet risk

• Diff search for S-prefixed 56-char secrets shows only synthetic throwaway fixtures.
• Defaults remain testnet-first; any new mainnet path is behind assertNetworkAllowed(...).

Risk / follow-ups

None. This PR consists strictly of documentation updates and reference links with zero impact or risk to runtime execution logic.