feat(cli): add --v8-flags plumbing to configure V8 at startup (#130) (#131)

* feat(cli): add --v8-flags plumbing to configure V8 at startup (#130)

Obscura had no way to pass flags to V8, so users hitting OOM on pages
larger than ~5MB had no escape hatch. Adds a generic passthrough rather
than a one-off heap knob, so future V8 tuning needs no new code.

- New `obscura_js::set_v8_flags` (Once-guarded, empty-string no-op).
- Global `--v8-flags <FLAGS>` arg with `allow_hyphen_values = true`,
  applied once in `main` before any isolate is created.
- Usage: `obscura --v8-flags "--max-old-space-size=4096" fetch <url>`.

Closes #130.

* test(cli): expand --v8-flags coverage with normalize helper

Extract normalize_v8_flags() from main() so the trim/empty-check logic
is unit-testable without needing to invoke V8. Adds parse-coverage tests
for --v8-flags with serve/scrape subcommands and empty-string values,
plus pure-function tests for the normalizer.

crates/obscura-js/src/lib.rs

@@ -4,7 +4,9 @@ extern crate html5ever;
 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;
+pub use v8_flags::set_v8_flags;

crates/obscura-js/src/v8_flags.rs

@@ -0,0 +1,37 @@
+use std::sync::Once;
+
+static INIT: Once = Once::new();
+
+/// Apply user-supplied V8 flags exactly once, before the first isolate is
+/// created.
+///
+/// `flags` is a raw V8 flag string in the same form V8/Chromium/Node accept
+/// (e.g. `"--max-old-space-size=4096 --max-semi-space-size=64"`). An empty or
+/// whitespace-only string is a no-op and does not consume the one-shot guard,
+/// so a later non-empty call still takes effect.
+///
+/// V8 ignores `set_flags_from_string` once the platform is initialized, so the
+/// first non-empty call must run before any `JsRuntime` is constructed.
+/// Subsequent calls are silently dropped.
+pub fn set_v8_flags(flags: &str) {
+    let trimmed = flags.trim();
+    if trimmed.is_empty() {
+        return;
+    }
+    INIT.call_once(|| {
+        deno_core::v8::V8::set_flags_from_string(trimmed);
+    });
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn empty_is_noop() {
+        // Must not panic and must not consume the Once guard.
+        set_v8_flags("");
+        set_v8_flags("   ");
+        set_v8_flags("\t\n");
+    }
+}