Add browser location bridge

[tnsardesai]

summary

• accept generation-ordered browser location bundles only for the active lease epoch
• move epoch transitions to an instance-JWT-authenticated lifecycle reset endpoint with compare-and-swap ordering
• persist desired state, reapply it after image API/Chromium restarts, and serialize cancellation so stale generations cannot mutate after a newer acceptance
• update /etc/localtime atomically and require browser/renderer/network/ICU component convergence before marking a bundle applied
• block all internal location commands on customer CDP while preserving standard target-scoped Emulation commands
• expose bounded-cardinality acceptance, retry, rejection, failure, latency, and component metrics
• retain TZ startup compatibility while new launchers initialize /etc/localtime and remove TZ before Chromium exec

tests

• make test-unit
• go test -race ./lib/cdpclient ./lib/devtoolsproxy ./lib/metrics ./cmd/api/api ./cmd/chromium-launcher
• go vet ./lib/cdpclient ./lib/devtoolsproxy ./lib/metrics ./cmd/api/api ./cmd/chromium-launcher
• focused proxy security, epoch ordering, durable restart, Chromium restart, metrics, and launcher compatibility tests
• CI headful/headless image builds and full server e2e suite passed (run 35309264560)

Runtime location application depends on the browser artifact from kernel-browser #92.