release: prepare ExcelAlchemy 2.2.6

[ruicore]

Summary

This PR prepares the 2.2.6 release of ExcelAlchemy.

It continues the stable 2.x line with stronger result-object guidance, a copyable FastAPI reference project, stronger release smoke verification, and clearer codec fallback diagnostics, while keeping the public import/export workflow API stable.

What changed

Result objects and API integration

• add docs/result-objects.md
• document ImportResult, CellErrorMap, and RowIssueMap as first-class integration surfaces
• show how to expose structured import feedback through backend APIs
• connect getting-started, public API, showcase, and README entry points to the result-object guide

FastAPI reference project

• add examples/fastapi_reference/
• split the example into:
  • route layer
  • service layer
  • storage layer
  • schema layer
• add a dedicated README for the reference project
• add a captured output artifact for the reference project

Smoke and release verification

• strengthen scripts/smoke_package.py
• verify:
  • successful import
  • failed import
  • structured cell_error_map payloads
  • structured row_error_map payloads
  • export upload path
• extend example smoke coverage so the FastAPI reference project is exercised directly

Diagnostics polish

• improve codec fallback warnings for invalid workbook input
• include clearer field context in warnings
• reduce low-value exception noise in fallback diagnostics

Compatibility notes

• no public import/export workflow API was removed
• ImportResult, CellErrorMap, and RowIssueMap remain stable public result objects in 2.x
• the FastAPI reference project is additive guidance and does not change the public API surface
• storage=... remains the recommended 2.x backend configuration path

Verification

• ./.venv/bin/ruff check .
• ./.venv/bin/uv run pyright
• ./.venv/bin/pytest -q
• ./.venv/bin/python scripts/smoke_package.py
• ./.venv/bin/python scripts/smoke_examples.py
• ./.venv/bin/python scripts/generate_example_output_assets.py

Release intent

After merge, this branch is intended to be tagged and published as:

• v2.2.6