refactor(api)!: drop Timing class; emit signed list[int] from get_raw_timings

[balloob]

Summary

Replaces the Timing dataclass with a flat list[int] return type on Command.get_raw_timings(), using the signed-microsecond convention (positive = mark, negative = space) shared by ESPHome, IRremoteESP8266, and most Python IR tooling.

Closes #15.

Why

• Less memory pressure. 34 Timing allocations per NEC call × 39 commands = 1326 dataclass instances for the bundled LG TV set alone. The new representation stores only ints (small ints are cached).
• Matches the ecosystem. ESPHome remote_transmitter, IRremoteESP8266 sendRaw, LIRC, and Flipper .ir all use flat lists; home-assistant/core consumers already flatten pairs on every call.
• Removes the low_us=0 smell. The trailing end pulse is now a single positive int instead of a fake-zero pair. See the previous NECCommand at commands.py:111 / :125.
• Prior art. rf-protocols made the same switch in PR #2 / commit 846d351; downstream core PRs (core#168447, core#168448, core#168450) all got shorter.

Breaking change

• infrared_protocols.Timing is removed.
• Command.get_raw_timings() returns list[int] (signed µs) instead of list[Timing].

Downstream integrations become:

# Before (esphome)
timings = [
    interval
    for timing in command.get_raw_timings()
    for interval in (timing.high_us, -timing.low_us)
]
# After
timings = command.get_raw_timings()

# Before (smlight — unsigned)
timings = [
    interval
    for timing in command.get_raw_timings()
    for interval in (timing.high_us, timing.low_us)
]
# After
timings = [abs(t) for t in command.get_raw_timings()]

Draft

Stacked on flipper-ir-loader. Depends on that branch landing first; target base will move to main once the parent merges. Release as 2.0.0 because of the signature break.

Test plan

• pytest — existing test_commands.py rewritten for list[int], all 18 tests pass
• ruff check + ruff format --check
• Downstream: home-assistant/core branch migrating lg_infrared, esphome, smlight, and kitchen_sink consumers (not yet pushed)