fix(nif): double-buffer the tap-handle registry (Android + iOS)

[GenericJam]

Problem

clear_taps (called every render) reset the tap-handle count to 0 and then
re-registered every handler in tree order. A high-frequency event — a finger
drag or scroll dispatched from the UI thread during a render — could
observe a transiently-small count and a half-rebuilt table, and get dropped.
The hazard grew with registration order: a widget registered late in the tree
(e.g. a Canvas after a row of Buttons) was the most likely to be missed.

This surfaced while wiring finger-drawing on a real device: drags onto a canvas
intermittently vanished depending on what else was on screen.

Fix

Double-buffer the tap-handle registry. register_tap builds into the inactive
table while readers keep resolving the last committed (active) table;
set_root swaps them atomically under tap_mutex. A concurrent event now always
resolves against a complete table on either side of the swap — never a torn,
mid-rebuild one.

• Android (mob_nif.zig): tap_tables[2] + tap_active / tap_active_count
  / tap_build_count; snapTap / throttleCheck / set_throttle_config read
  the active table; register_tap / clear_taps write the building table;
  set_root swaps under the mutex.
• iOS (mob_nif.m): tap_handles / tap_handle_next become a pointer+count
  to the active table (readers unchanged); register/clear use the building table;
  the pointer is repointed at set_root.

No API change — purely an internal correctness fix to event delivery.

Verification

• Compiles clean on both platforms (mob_nif exports unchanged: 95 on iOS).
• Verified on-device (moto g, Android, mob 0.7.3 via path-dep + explicit
  mix deps.compile mob --force): a finger-drag canvas routed 74 drag samples
  through the swapped table and rendered the stroke correctly — drags resolve
  through the active table across renders with no drops.

Docs (so users can track this)

• CHANGELOG.md [Unreleased] → Fixed.
• guides/troubleshooting.md: a path-dep mob_nif:log undef-at-boot entry —
  recompile the path-dep as its own step before deploy (and match the
  elixir_lib toolchain). Encountered while verifying this change on-device.

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