feat: add auto-wait and assertion commands

[tosh7]

Summary

• Auto-wait: Element-level stability detection before tap (replaces coarse hash-based retry for semantic selectors)
• Assertion: era assert command for UI state verification with polling + timeout, exit code 0/1 for shell scripting

Changes

Phase 4a: Auto-wait (wait.rs)

• wait_for_element(): Polls UI tree until target element is visible + enabled + frame-stable (2 consecutive polls match)
• tap_element_with_wait(): Pre-tap wait → tap → post-tap change detection
• Frame gains PartialEq for stability comparison
• handle_tap uses auto-wait for --text/--type selectors; coordinate mode retains existing tap_with_retry

Phase 4b: Assertion (assertion.rs)

• Selector enum: Text, Type(String, Option<u32>), Ref(u32)
• Condition enum: Visible, Hidden, Enabled, Disabled, TextEquals, TextContains, Count
• assert_condition(): Polling loop (500ms interval, configurable timeout)
• era assert CLI with subcommands: visible, hidden, enabled, disabled, text, count
• SelectorArgs with #[command(flatten)] for reusable --text/--type/--index/--ref options

Usage examples

# Wait for element then tap (auto-wait)
era tap -d <device> --text "Login"

# Assert element is visible (5s timeout)
era assert -d <device> visible --text "Welcome"

# Assert element hidden (loading spinner gone)
era assert -d <device> --timeout 10000 hidden --text "Loading..."

# Assert text content
era assert -d <device> text --ref 5 --equals "Order Complete"

# Assert button count
era assert -d <device> count --type Button --expected 3

# Shell scripting
era assert -d $UDID visible --text "Login" && era tap -d $UDID --text "Login"

Test plan

• All 246 tests pass (143 lib + 18 coordinate + 56 regression + 12 retry + 17 scale)
• 33 assertion unit tests covering all conditions, selectors, and edge cases
• Auto-wait stability detection tests
• CLI parse tests for assert subcommands