Implement per-pointer gesture cancellation

[nicodes]

Self-verification evidence (Team No Review)

• Issue / criteria: fieldsofrevik#144 — D-04 per-pointer ownership, GUI acceptance, cancellation/focus loss, pinch ordering, startup double tap, synthesis, docs/arity, native/web package behavior.
• Correction (fieldsofrevik#144): after native bottom-layer merge and trivial replay, source is aviorstudio/gd-gesture, a04-behavior, exact head 590a1ffb1b01785bbca1eabc8b88697d5f67b187 on main. The earlier pre-merge head was superseded.
• Decision consumed: approved D-04. Caller selected absolute pinch scale on 2026-09-12: current distance / distance when the two-pointer pinch began. Backward compatibility waived.
• Criterion evidence: PASS — independent simultaneous drags retain pointer ownership; moved touch is stored before absolute pinch calculation (2.0 then 3.0 controls); first 1ms tap is not double; distant/late taps are not double; timely nearby taps are; pinch participants do not also tap.
• Criterion evidence: PASS — screen cancel, focus loss, pointer removal and teardown emit cancellation; later release cannot tap/swipe and delayed long press remains canceled. GUI-accepted input is ignored. Synthetic mouse (device=-1) is ignored while native mouse remains active.
• Docs/API: PASS — README correction names originating issue, uses pointer_dragged/pointer_canceled, and compiles signal callbacks at actual arity (readme_signal_contract_test, 7 reachable assertions).
• Versions/commands: Godot 4.7.2.stable.official.ed1daf0bf; Python 3.13; Linux; Firefox via playwright-cli 0.1.18; GODOT_BIN=... ./tests/test.sh; deterministic two-build comparison; GODOT_BIN=... ./tests/package_lifecycle.sh; Web export; python3 -m http.server 8894; playwright-cli open/run-code/screenshot/console.
• Reachability: runner controls 12/12 PASS; Godot suites emitted exact single sentinels with assertions 2, 16, 6, and 7.
• Artifact identity: deterministic exact-head ZIP SHA-256 c124429c242a906ba5b4e7ebde9fb7b63b296c4a4063b3d571e8b5cd97325b76; installed-tree SHA-256 ebe05cd95433ac6274c7ff6f1dece9541587c93ffff1b81d1863b6252ea0c4b6.
• Browser: packaged Web export loaded title gd-gesture web playground, canvas 1280x720, click emitted tap • event 1 • {position:(640,360), index:0}, and console had 0 errors. Firefox reported only the upstream WebAssembly deprecated-try warning.

• Exact-head CI: PASS — run 34740314592 at 590a1ffb1b01785bbca1eabc8b88697d5f67b187 after the bottom-layer merge/replay.
• Untested/residual: physical mobile multi-touch and non-Linux native platforms are outside the approved D-14 runner matrix. Release/GDAM publication and clean registry install are post-merge and block issue completion.