feat(router): tighten routing match semantics

[typelicious]

What changed

• tighten and match semantics so combined fields behave as cumulative constraints
• keep explicit as the OR path for composed rules
• add routing tests that lock the intended semantics in place
• document the rule-composition behavior in architecture docs

Why

• the routing review found that combined match blocks were easier to read as AND than the old implicit any-field behavior
• this reduces accidental matches and makes layered routing easier to reason about
• it keeps future rule growth safer without adding new routing layers

How verified

• python3 -m compileall foundrygate tests
• PYTHONPATH=. ./.venv-check-313/bin/pytest -q tests/test_routing.py
• ./.venv-check-313/bin/ruff check foundrygate/router.py tests/test_routing.py docs/ARCHITECTURE.md CHANGELOG.md
• /usr/bin/git diff --check