perf(cdp): scope the V8 lock per connection to run connections in parallel

With every connection on its own thread (previous commit), the process-wide V8
lock served no safety purpose across connections yet forced all of them to
serialize on one mutex, so throughput did not scale with concurrency. Move the
lock into CdpContext so each connection has its own: a connection's nav task and
command dispatch still coordinate on one page's isolate at a time on its thread,
but different connections no longer block each other. Delete the now-unused
process-wide obscura_js::v8_lock.

Measured on a local CPU-bound fixture, page throughput stays flat (~75/s from
concurrency 1 to 8) with the global lock and scales to about 2.5x at concurrency
8 with the per-connection lock, with no crash at any concurrency. Obstacle
course 33/33, obscura-cdp 76/76.

Refs #430.

crates/obscura-js/src/lib.rs

@@ -6,7 +6,6 @@ pub mod module_loader;
 pub mod runtime;
 pub mod ops;
 pub mod v8_flags;
-pub mod v8_lock;
 pub mod markdown;
 
 pub use markdown::HTML_TO_MARKDOWN_JS;