feat: positional subject arguments for Factory, ContextProvider, Alias (API-5)

[lesnik512]

The accepted API-5 item from the 2026-07-05 3.0 UX research, extended by maintainer ruling to all three registerable providers: each provider's subject parameter — Factory(creator), ContextProvider(context_type), Alias(source_type) — moves ahead of the * and becomes positional-or-keyword. Factory(UserService) is the new shortest registration; every existing keyword call site keeps working unchanged (no deprecation, no warnings, no /).

• Docs/README teach the positional spelling throughout; each provider page states the new signature once with a one-line keyword-form mention; to-2.x.md's keyword-only note is version-stamped ("Since 2.27").
• architecture/providers.md promoted in the same PR.
• New tests: positional construction resolves through a container for all three; double-pass raises Python's native TypeError. Existing keyword call sites left untouched as living back-compat proof.
• Gates: just test-ci (100% line coverage, 311 passed), just lint-ci, just docs-build --strict, just check-planning — all green.

Design bundle (rationale home): planning/changes/2026-07-08.01-positional-subject-args.md

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