fix: Share single Deno/V8 worker thread

[jonmmease]

Summary

Share a single Deno/V8 worker thread across all VlConverter instances, fixing segfaults caused by multiple V8 isolates coexisting in the same process (#206). Adds font configuration version tracking so dynamically-registered fonts propagate to long-lived canvas contexts, and a worker respawn mechanism so the process can recover from worker thread failures.

Motivation

Previously, each VlConverter::new() spawned a dedicated worker thread with its own V8 isolate. This caused segfaults when multiple VlConverter instances existed simultaneously (#206), because V8 isolates cannot safely coexist in the same process.

Changes

Core architecture (vl-convert-rs/src/converter.rs, +214/-118)

• New VlConverterRuntime struct holds a Sender<VlConvertCommand> and the worker thread JoinHandle
• VL_CONVERTER_RUNTIME is a static Mutex<Option<VlConverterRuntime>> so the worker can be respawned if it exits
• get_or_spawn_sender() checks if the worker is alive via JoinHandle::is_finished(); if the worker has exited, it spawns a fresh one transparently
• spawn_worker_thread() extracted as a standalone function (was previously inlined in lazy_static!)
• VlConverter no longer caches a sender — each method call goes through get_or_spawn_sender(), ensuring it always talks to a live worker
• Commands dispatched via bounded mpsc channel (capacity 32), processed sequentially by the worker

Error resilience

• VlConvertCommand::send_error() method forwards errors to the command's oneshot responder regardless of variant type
• refresh_font_config_if_needed errors in the worker loop are caught, sent to the current command's responder, and the loop continues (previously the ? operator killed the worker thread)

Font version propagation

An AtomicU64 counter (FONT_CONFIG_VERSION) is incremented when fonts are registered via register_font_directory(). The worker thread and individual canvas contexts each track the last-seen version, refreshing their font databases lazily when a mismatch is detected. This ensures dynamically-registered fonts are available even in long-lived canvas contexts that Vega caches internally.

op_canvas_set_font now calls refresh_canvas_fonts_if_needed, matching the pattern in the other text ops (measureText, fillText, strokeText, etc.). This ensures newly-registered fonts are available when Vega sets a font, not just when it measures or renders text.

Supporting changes

• SharedFontConfig changed from tuple struct to named fields (adds version: u64)
• CanvasResource::new() accepts initial font config version
• Canvas2dContext::update_font_database() method for hot-swapping fonts on existing contexts

Review Tour

• vl-convert-rs/src/converter.rs — The core architectural change. Search for fn spawn_worker_thread() for the worker loop, fn get_or_spawn_sender() for the respawn mechanism, and impl VlConverter for the simplified public API. VlConvertCommand::send_error() is near the enum definition at the bottom.
• vl-convert-rs/src/text.rs — Global FONT_CONFIG_VERSION atomic counter, incremented in register_font_directory
• vl-convert-canvas2d-deno/src/ops/text.rs — Per-canvas refresh_canvas_fonts_if_needed() helper that propagates font changes from the shared config to individual canvas contexts. Called in 6 text ops (including op_canvas_set_font).
• vl-convert-canvas2d-deno/src/lib.rs — SharedFontConfig struct evolution (tuple to named fields with version)
• vl-convert-canvas2d-deno/src/ops/mod.rs — op_canvas_create captures font version
• vl-convert-canvas2d-deno/src/resource.rs — CanvasResource gains font_config_version field
• vl-convert-canvas2d/src/context/mod.rs — update_font_database() method on Canvas2dContext

Testing

Existing test suite passes (117 tests via cargo test -p vl-convert-rs -- --test-threads=1). The architectural change is transparent to the public API — all existing conversions produce identical output.

New test test_font_version_propagation verifies that register_font_directory increments the version counter and that the worker thread survives the font config refresh, confirming the 3-layer version propagation mechanism works end-to-end.