feat(time-offset): add Time Offset block for live PTS shifting

[srperens]

Summary

• New builtin.time_offset block: a single identity element whose src pad has a GStreamer pad-offset applied. Media-agnostic (Generic in/out pads), so the same block works on audio or video branches.
• offset_ms (Float, signed) is live-updatable. A small interceptor in update_element_property catches it before the default property-set path and applies pad.set_offset() directly, then triggers pipeline.recalculate_latency() so downstream sinks resync.

Why

Two use cases drove this:

• Lipsync — drop a Time Offset block between an input and a mixer to align audio against video (or vice versa).
• Per-branch latency shaping — give different output branches different latency budgets (e.g. WHEP PGM gets full pipeline latency, WHEP multiview is shifted to play out earlier relative to PGM).

Design notes

• Positive offset_ms shifts buffers later (delay), negative shifts them earlier (advance). Negative is accepted in the API, but with live sources only positive is physically meaningful — the pipeline cannot emit data ahead of the source. Documented in the property description.
• The interceptor is keyed on (prop_name == "offset_ms", element_id ends_with ":offset_identity"). Coupling to the block's PropertyMapping and element naming is documented at the function site.
• The block reuses the existing exposed-property infrastructure for persistence, WebSocket events, properties-panel UI, etc. — no new API endpoints, no new event variants.

Test plan

• cargo test --lib time_offset — 6 unit tests covering definition shape, live apply (positive, negative, non-matching element, non-matching property), and initial offset on build
• cargo test --test openapi_test — snapshot unchanged (no new API types)
• cargo test --test pipeline_lifecycle_test — leak regression still passes
• cargo check and clippy pre-commit hooks clean
• Manual: create flow with Time Offset between input and mixer, verify lipsync drag in properties panel updates pipeline live without restart

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