config: fix yaml tags for FrontingSNIs / SNIConfig

[myleshorton]

Summary

Config.Providers[*].FrontingSNIs has been silently empty for every consumer that parses fronted.yaml.gz with this library — a yaml-tag mismatch the linter doesn't catch.

The upstream generator (lantern-cloud/cmd/update_masquerades) writes the file using the older getlantern/fronted types, which carry no yaml struct tags. yaml.v3 falls back to lowercased-concatenated field names there, producing:

frontingsnis:
    ir:
        usearbitrarysnis: true
        arbitrarysnis:
            - python.org

This package's tags expected the snake_case form (fronting_snis, use_arbitrary_snis, arbitrary_snis), so yaml.Unmarshal silently leaves the fields zero-valued. The runtime fronting client's arbitrary-SNI cover code path has been dead for every client.

Fix

Aligns the tags with the wire format. Three tags change in config.go:

-FrontingSNIs map[string]*SNIConfig `yaml:"fronting_snis"`
+FrontingSNIs map[string]*SNIConfig `yaml:"frontingsnis"`

-UseArbitrarySNIs bool     `yaml:"use_arbitrary_snis"`
-ArbitrarySNIs    []string `yaml:"arbitrary_snis"`
+UseArbitrarySNIs bool     `yaml:"usearbitrarysnis"`
+ArbitrarySNIs    []string `yaml:"arbitrarysnis"`

No migration cost: the older format is the only one ever produced.

Verification

Parsing the live fronted.yaml.gz from getlantern/fronted/main with the fixed library:

provider=akamai FrontingSNIs groups: 3 Masquerades: 1000
  cn:      useArbitrary=false count=2570
  default: useArbitrary=true  count=4638
  ir:      useArbitrary=true  count=0
provider=cloudfront FrontingSNIs groups: 0 Masquerades: 1000

Before the fix all three groups parsed as empty (FrontingSNIs map was nil because the top-level key didn't match either).

Consumer impact

• Runtime fronting client (domainfront.ExpandedProvider) now sees arbitrary-SNI cover groups — this is the user-visible behavior change.
• radiance#488's scanner now reads these via SNIsForProvider's new FrontingSNIs union, so the new IR-validated SNIs (queued in lantern-cloud#2799) make it into the scanner pool.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes a YAML tag mismatch so Config.Providers[*].FrontingSNIs (and nested SNIConfig fields) correctly unmarshals from the fronted.yaml.gz wire format, restoring the arbitrary-SNI cover configuration path for consumers.

Changes:

• Update Provider.FrontingSNIs YAML tag to match the lowercase-concatenated key emitted upstream (frontingsnis).
• Update SNIConfig YAML tags to match the upstream-emitted keys (usearbitrarysnis, arbitrarysnis).
• Add an in-code comment documenting why these tags must match the wire format exactly.

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