Establish pre-engineering validation foundation - #1

What changed

Executable input and transaction specification

• adds a platform-independent C++20 input ownership and interaction-session core
• tests pen exclusivity, two-finger ownership, remaining-finger blocking, mixed pen/touch sequences, lifecycle reset, and complete-vs-abort terminal rules
• runs the same core tests on Ubuntu and macOS

Trustworthy Android input measurement

• defines a versioned JSONL contract with session, event, and explicit drop records
• snapshots MotionEvent primitive fields on the UI thread, then encodes and writes on a bounded background queue
• records current and historical samples, action pointer identity, tool/source/device/display data, pressure/tilt/distance, raw/local coordinates, motion ranges, device fingerprint, and embedded Git SHA
• adds JVM tests for writer flush/close and deterministic queue-overflow drop reporting
• adds an offline validator and summary tool that distinguishes ACTION_CANCEL from API 33+ FLAG_CANCELED and isolates pointer ID reuse by downTime
• documents a reproducible 14-scenario Xiaomi Pad 7 Ultra test protocol

Source and build-risk controls

• separates immutable research candidates from the validated build lock
• verifies candidate SHAs against official remotes
• audits the upstream source contracts the architecture depends on
• inventories the Android target dependency surface
• adds a fail-fast sysroot preflight that prevents host/Homebrew contamination and blocks Inkscape's non-cross-safe gtkmm/glibmm fallback
• expands Mac environment diagnostics and locked SDK package guidance

Evidence from the current PR head

Head SHA: a0bb0a945c35370e272224fe90a302ddb3155045

All current checks pass:

• Scaffold check
• Pre-engineering checks / Ubuntu
• Pre-engineering checks / macOS
• Source candidate official reachability
• Upstream source contract audit
• Input probe lint, JVM tests, debug APK build, and APK content verification

Final workflow artifacts:

• input-probe-debug artifact digest: sha256:89908d32ae40ac74437f6eb946b9f2cb5e88b80398759ee12caeaad14316dec6
• extracted APK SHA-256: 8cd10cbc442c69800063b1367ac69a522de49a65a14cab338e797b8c3affe579
• input-probe-reports artifact digest: sha256:8b1b19c4e4b3799a6286bb60bea49419f8d04f429e19fc4a52031041c6246c8c
• upstream contract audit artifact digest: sha256:5242a229b37237801dfaf8232a6801b4c9843c411cd9312693fa056e3e59011e

The upstream audit materialized the exact Inkscape, GTK, and Pixiewood candidate SHAs from their official remotes and passed every defined architecture contract.

Why

The repository previously contained a useful plan but not enough executable evidence. This PR establishes the engineering and measurement baseline required before modifying GTK or Inkscape.

Important boundaries

• sources.candidates.toml is not a validated build lock.
• the C++ input core is an executable specification, not the final GTK adapter.
• the probe measures raw Android events; interpretation remains offline.
• no Inkscape or GTK patch is introduced in this PR.
• P0 passing does not prove Xiaomi device behavior; P1 requires real-device capture with droppedRecords = 0.
• the PR remains draft pending review of the foundation and completion of P1 device evidence.