perf(procaptcha): start detector assignment at page load

[HughParry]

Measured with headless Chromium against the demo pages:

	page load → frictionless complete
production (3.6.76, no assign step)	1744 ms
staging (3.7.0)	4928 ms

Staging waterfall:

 466 ms  procaptcha.bundle.js
 632 ms  ┐ 7 sequential dynamic-import levels (~700 ms)
1166 ms  ┘ ProviderApi chunk finally loaded
1513 ms  detector/assign starts          <-- nothing before this needed it
2448 ms  …935 ms to download the bundle
3817 ms  frictionless starts
4928 ms  done

Change

Nothing in the assign request depends on React, i18n or the widget config. It needs the site key (a DOM attribute), the environment (a build-time constant) and the IP-mode flags (DOM attributes) — all available at ~0 ms. The entry now starts it as soon as it has read them, and customDetectBot claims the in-flight promise instead of issuing its own.

The ProviderApi + provider-selector chunks now download in parallel with React/i18n rather than after them, so assign should start around 400–500 ms instead of 1513 ms.

Safety

• Dynamic import, so the provider selector and API client stay out of the entry chunk. Entry grows 17,985 → 18,387 bytes (+402).
• Fire-and-forget. A failed prefetch is indistinguishable from no prefetch — the existing path still resolves a provider itself, so this cannot turn a working session into a broken one. A no-op .catch is attached so it never surfaces as an unhandled rejection in the host page, while the real consumer still sees the rejection and falls back.
• Single-use cache, keyed on (environment, ipMode, siteKey). A retry is retrying because the pinned pronode failed, so it must not reuse that pin; and a second widget with different IP flags must not claim another widget's assignment. Both are covered by tests.

Verification

• procaptcha-frictionless + procaptcha-bundle typecheck clean
• 24 tests pass, including 6 new ones for the cache and the pre-existing customDetectBot suite (which this PR modifies)
• built bundle confirmed to contain startDetectorPrefetch in the entry chunk — if it landed in a lazy chunk the change would silently do nothing

IpMode is derived via Parameters<typeof getProcaptchaRandomActiveProvider>[1] rather than importing @prosopo/load-balancer, avoiding a new dependency plus tsconfig project reference for one type alias.

🤖 Generated with Claude Code

https://claude.ai/code/session_01XJ7KiTDKiu3mxQ4iuLFK2y