docs: define Pulse UI Phase-1 architecture, API, and MVP milestones

[michaelgrilo]

Motivation

• Start Phase 1 (DEFINE) for a single-engine, pixel-identical Rust UI workspace by recording architecture, public API, and MVP milestones before implementation begins.
• Provide a concrete, test-driven plan (requirements checklist + DTI milestones) so later phases can follow a test-first implementation path.

Description

• Added docs/Architecture.md that describes vision, goals/non-goals, deterministic rendering model, reactive/invalidation model, layout/text plan, event/focus model, web kernel + lazy-wasm strategy, resumability plan, and a requirements checklist.
• Added docs/API.md that drafts the public API surface for all planned crates (pulse_reactive, pulse_ui_core, pulse_layout, pulse_render, pulse_text, pulse_platform, pulse_web, pulse_macros) and the feature = "resumable" snapshot API.
• Added docs/MVP.md with DTI-aligned milestones (MVP0–MVP3), required tests per milestone, acceptance criteria, and run instructions for later phases.
• Bootstrapped workspace skeleton directories under crates/ and examples/ and added placeholder .gitkeep files to establish the planned crate and example layout.

Testing

• Verified repository layout using find . -maxdepth 3 -type f | sort to confirm new docs and placeholder files are present.
• Verified working tree and recorded changes with git status --short and created a commit which succeeded.
• No crate compilation or unit tests were added or run in this DEFINE phase, so cargo test was not executed yet.
• Next phase will add the test-first suites referenced in docs/MVP.md and docs/Architecture.md and run them as part of implementation.

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Codex Task