deps: restore sing-box-minimal v1.12.21-lantern (Android bootstrap deadlock)

[myleshorton]

Summary

Bumps sing-box-minimal back to v1.12.21-lantern. This was the version on main until #8655 "Save last server location" accidentally downgraded it to v1.12.19-lantern during review churn.

v1.12.21-lantern contains 9c79c311 ("fix: make initial remote rule-set fetch non-fatal"), which turns the Android bootstrap deadlock (`"no available network interface"` during initial rule-set fetch) from a fatal libbox startup error into a WARN + retry-after-start. Without it, nightly builds from main fail to connect on any smart-routing country.

Diagnosis

Same user (Alexander, Bulgaria), same device (28daebb937d60563), same 9.0.25 version, different builds:

Ticket	Build	sing-box-minimal	rule_set_remote.go line	Outcome
#172722	Nightly (from main, post-#8655)	v1.12.19-lantern	235 (fetch() error path)	ERROR Failed to start libbox service — tunnel never starts
#172795	Beta v9.0.25-beta-android (cut before #8655)	v1.12.21-lantern	113 (StartContext warn)	WARN initial rule-set fetch failed, will retry after start — tunnel starts cleanly

Same smart-routing-bg-common-direct fetch failure in both logs; only the sing-box-minimal version differs.

Follow-ups

• getlantern/radiance PRs don't fail silently when disk is full #423 and Consider adding SPDY support to all proxies. #424 (Go-level rule-set pre-download) closed — redundant with the sing-box-minimal fix.
• getlantern/lantern-cloud Migrate from gost to gb #2595 retires the dns_ruleset_host_bypass feature flag and its DNS bypass rule — verified empirically that routing rule-set hostnames to dns_local does not help (same no available network interface error), since the problem is sing-box's dialer having no registered interfaces, not the DNS routing decision.

go mod tidy cleanup

Tidy also dropped stale go.sum entries for superseded radiance and lantern-box pseudo-versions and removed the unused getsentry/sentry-go indirect (left behind after #8665 "remove sentry").

Test plan

• go build ./... clean
• go mod tidy clean
• Next Android nightly: verify Alexander's Bulgaria build connects (same user can retest)
• Smart-routing countries (MO, CN, RU, BG) all connect on first launch without the bootstrap deadlock loop

[Copilot]

Pull request overview

Restores the sing-box-minimal replacement version to v1.12.21-lantern to address the Android bootstrap deadlock during the initial remote rule-set fetch, and cleans up module metadata from go mod tidy.

Changes:

• Update go.mod replace directive to use github.com/getlantern/sing-box-minimal v1.12.21-lantern.
• Update go.sum accordingly and drop stale sums for no-longer-referenced module versions (including removed sentry-go indirect entries).

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated no comments.

File	Description
go.mod	Restores the sing-box replacement target to sing-box-minimal v1.12.21-lantern and removes an unused indirect requirement.
go.sum	Synchronizes sums with the restored dependency version and tidies out unused/stale entries.

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