docs(groq): S6 polish — streaming caveat, surcharge note, API reference (#69)

[stackbilt-admin]

Implements S6 (final) of the Groq built-in-tools sprint (#69), on top of the merged S5 (#73). Docs-only — every acceptance criterion in #69 is already satisfied across S0–S5; this rounds out documentation completeness.

Changes

• Streaming caveat — builtInTools is accepted on streaming requests and the search runs server-side, but the streaming path emits content deltas only; structured metadata.builtInToolResults / metadata.reasoning are surfaced on non-streaming generateResponse only. Documented, not gated — a hard reject would block legitimate streamed answers that used search internally (the search result is folded into the streamed content). README + CHANGELOG.
• CreditLedger surcharge note — the Cost Tracking section now states that built-in tool surcharges (web search ~$5/1k req, code interpreter ~$0.18/hr, browser automation ~$0.08/hr) are billed per-use and not token-tracked in v1 (the issue's explicit S6 ask).
• API Reference — added BuiltInTool / BuiltInToolType / BuiltInToolResult to Key Types; updated LLMRequest (builtInTools), LLMResponse (metadata), and the GroqProvider class row.

Acceptance criteria (#69) — all met

AC	Where
builtInTools on openai/gpt-oss-120b works; cost noted as not token-tracked	S4 + this PR
Same on groq/compound	S4
builtInTools on llama-3.3-70b-versatile → ConfigurationError naming capable models	S4
MODELS.GROQ_COMPOUND exported; catalog RESEARCH tag	S3
Citations accessible programmatically (metadata.builtInToolResults)	S5
Existing function-tool callers unchanged	green throughout
Golden fixtures committed (compound + gpt-oss)	S0
Tests: request shape, per-model rejection, citation extraction	S4 + S5

Note: the public built-in-tool identifier is normalized to web_search (Groq Compound's vocabulary), not browser_search as written in the issue body; the adapter translates to browser_search on the gpt-oss path. Flagged on #70 and carried through.

Build clean (docs-only; no test change). This completes issue #69.

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