Add AnimationKeyMode and open-frame shorthand; update parsing, sampling, optimization, and static emission

[TiernanDeFranco]

Motivation

• Introduce a per-key mode to distinguish open (one-sided) keys from closed keys so open-frame shorthand can be expressed and handled.
• Ensure optimization and sampling respect open keys so redundant-key removal and value sampling remain correct for open segments.
• Propagate key mode through the parser and track-building to allow .panim to express open frames with a compact shorthand ([FrameN?]).

Description

• Added AnimationKeyMode enum with Open and Closed variants and added a mode field to AnimationObjectKey with Closed as the default and adjusted Default implementations accordingly.
• Updated parser to accept an open-frame shorthand by returning (frame, key_mode) from parse_frame_header when a frame header ends with ?, and threaded key_mode through parse_frame_block, parse_object_block, parse_object_field_action, and FrameAction::Field.
• Threaded key mode into track building by storing per-frame mode in TrackKey and setting key.mode for keys that originate from frame headers; insertion defaults keys to Closed where appropriate.
• Modified optimization and sampling logic so optimize_animation_track_keys skips tracks that contain any non-Closed keys, and sample_track_value_from_keys returns None for frames that fall after an Open key segment start (except at the exact open key frame).
• Updated static emission (emit_static_animation_const) to include key mode in generated AnimationObjectKey constants and added emit_key_mode helper.
• Updated tests and helpers across crates to account for the new mode field and added new tests: optimize_track_keys_does_not_optimize_tracks_with_open_keys, sample_track_value_returns_none_after_open_key_segment_start, and parses_open_frame_header_shorthand.

Testing

• Ran cargo test for the modified crates, including parser and animation unit tests (perro_animation and pipeline tests); all automated tests passed.

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Codex Task