feat: add --continueOnError flag to skip a hung/crashing document

[sajeeshnaniyil]

Adds a --continueOnError flag (also -C, also ACE_CONTINUE_ON_ERROR=1) that turns a per-document failure inside the HTML checker's pMap from fail-fast into log-and-skip. Default behavior is unchanged.

Closes #465.

What changes

• packages/ace-core/src/checker/checker-chromium.js — the pMap mapper now wraps checkSingle() in a try/catch when continueOnError is set. Failing docs get a winston.warn("Skipping document …") and yield null; nulls are filtered from the results before consolidate() sees them.
• packages/ace-core/src/checker/checker.js — accepts and forwards the flag.
• packages/ace-core/src/core/ace.js — computes continueOnError from options.continueOnError || process.env.ACE_CONTINUE_ON_ERROR === '1', following the same shape as doNotReportMedia.
• packages/ace-cli-shared/src/index.js — exposes -C, --continueOnError in the CLI help + meow flags.

Why

Some real-world EPUBs occasionally have a single content document that either hangs the axe runner or times out. Today that single per-document rejection propagates through pMap's default fail-fast and aborts the entire audit — no report is produced even when 100+ other documents completed fine. Consumers who'd rather trade a small amount of coverage for a completed report have no escape hatch.

This flag is that escape hatch, opt-in only.

For the environment story of what was hanging on our end, and how we ended up not needing this in production once we reverted our own overzealous Electron flags — see #465 and its follow-up comment. The flag is still broadly useful independent of our specific pathology.

Compat

• No signature break for existing callers — continueOnError is optional at every layer.
• No change to the report shape or the CLI's exit codes.
• Off by default, so daisy/ace's current fail-fast semantics are preserved unless someone explicitly opts in.

Tests

I skipped adding a unit test because there aren't any existing tests for checker-chromium.js / checker.js to build on, and simulating a per-document hang inside the pMap in isolation would need a fair bit of mock scaffolding for pMap + axeRunner. Happy to add one if you'd prefer — pointer to any preferred mocking pattern would speed that up.

Commit

Angular-style per CONTRIBUTING.md:

feat: add --continueOnError flag to skip a hung/crashing document