fix(config): deterministic and transactional state->config migration

[juzigu40-ui]

Summary

This PR hardens legacy state -> config bootstrap migration in desloppify/base/config.py to remove two reliability hazards present in snapshot 6eb2065:

• non-deterministic scalar precedence across multiple state files
• destructive partial migration when config.json persistence fails

What changed

• Added deterministic migration order via _state_files_for_migration().
  • state-*.json processed in stable lexical order
  • state.json processed after scoped files
• Switched migration to a two-phase flow:
  • Phase 1: merge legacy config in-memory only
  • Phase 2: write config.json; only after successful write, strip config from source state files
• Updated _migrate_single_state_file() to be non-destructive (merge-only)

Why this matters

Before this patch, migration could rewrite source state files before destination durability was guaranteed. On write failure (permissions, transient I/O, full disk), original embedded config could be lost while config.json was still missing. This patch prevents that failure mode.

It also stabilizes scalar precedence so equivalent inputs do not produce environment-dependent outcomes from filesystem iteration order.

Tests

Added regression tests in desloppify/tests/core/test_config.py:

• test_scalar_merge_order_is_deterministic
• test_save_failure_does_not_strip_source_state_config

Local verification

• uv run python -m pytest -q desloppify/tests/core/test_config.py
• uv run python -m ruff check desloppify/base/config.py desloppify/tests/core/test_config.py

[juzigu40-ui]

@peteromallet Added two regression tests that capture the practical impact discussed in #204:

1. deterministic scalar merge precedence across multiple state files
2. no source-state config stripping if config persistence fails

This keeps migration recoverable under write failure and deterministic across environments.