feat: add expect.* factories + consolidate arch/ module

[othercodes]

Summary

Two related improvements:

1. expect.* as unified entry point — thin factory methods on expect that wrap the existing constructors. Same fluent style for any subject, lazy imports keep expect light.
2. arch/ module consolidation — collapses 4 internal files into a single assertable.py, removing the entrypoint.py indirection that broke convention with the rest of the library.

What's new

Bridge

expect(value)                # → AssertableValue (generic, existing)
expect.json(data)            # → AssertableJson
expect.html(markup)          # → AssertableHtml
expect.mcp(payload)          # → AssertableMCP
expect.arch("myapp.domain")  # → AssertableArch | AssertableMultiArch

• Direct constructors (AssertableJson(...), etc.) remain available — zero breaking changes.
• Each factory does lazy import so from pyssertive import expect stays light.
• expect.arch has explicit signature (name, optional callback) for proper mypy/IDE support.

_MultiAssertableArch → AssertableMultiArch

The class was already exposed in public type signatures (return type of assert_arch, callback type) but prefixed with _ indicating internal. Renamed to match the Assertable* convention and added to __all__ so it's a first-class public type.

arch/ module consolidation

Before:
```
arch/
├── init.py
├── _chains.py
├── assertable.py (AssertableArch, AssertableMultiArch)
├── entrypoint.py (_AssertArch, assert_arch singleton)
├── graph.py (build_graph)
├── layers.py (AssertableLayers)
└── modules.py (AssertableModules)
```

After:
```
arch/
├── init.py (public re-exports)
├── _chains.py (graph traversal helpers — kept separate)
└── assertable.py (everything Assertable + _AssertArch + assert_arch + build_graph)
```

Why: the rest of the library follows "the Assertable class is the entry point" (cf. formats/json.py, formats/html.py, protocols/mcp/assertable.py). arch/ was the outlier with a separate entrypoint.py. Consolidating restores convention. The _chains.py graph traversal helpers stay separate (different responsibility — _ prefix correctly signals internal).

Test plan

• 5 thin bridge tests in tests/test_assertions.py — one per expect.* entry point, type-check only (behavior covered by the underlying suites).
• All existing tests pass after the rename and the arch consolidation.
• mypy strict clean (now 35 source files, down from 39).
• Coverage 100% maintained.
• ruff lint + format clean.
• README: new #### Unified entry point section in ### Expectations.

Backward compatibility

100%. No public APIs changed:

• AssertableJson(...), AssertableHtml(...), AssertableMCP(...), assert_arch(...) all continue to work.
• from pyssertive.arch import AssertableArch, assert_arch (the public path) unchanged.
• Internal from pyssertive.arch.entrypoint import ... / from pyssertive.arch.layers import ... / etc. would break, but nothing inside or outside the library used those paths.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud