feat: add Special Situations SDK workflows

[daniel-scrivner]

Summary

• add grouped and flat public Special Situations helpers
• support archive issues, detail/timeline, Copy-for-LLM markdown export, feed, calendar, stats, performance, and form lookup
• cover routes, escaping, raw markdown transport, and API-error preservation

Dependency

• archive issue helpers require datastream PR #1363 to be merged and deployed before production use

Validation

• cargo test
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps
• git diff --check
• two-pass adversarial GPT-5.5 High review

Boundary

• public Special Situations only; no internal-only data capabilities are exposed

[daniel-scrivner]

Adversarial review record

• Pass 1 (GPT-5.5 High): accepted the README correction from summary["markdown"] to summary["summaryMd"].
• Pass 2 (GPT-5.5 High): no material findings.
• Rejected feedback: none.
• Residual note: cargo fmt -- --check has broad pre-existing repository drift outside this focused change; touched behavior passed cargo test and rustdoc warnings-as-errors.

[daniel-scrivner]

Underwriting-pack adversarial review record

• Pass 1 (GPT-5.5 High): accepted README correction from unsupported situationId to top-level id.
• Pass 2 (GPT-5.5 High): no findings.
• Rejected feedback: none.
• Verification: cargo test, doc-tests, and diff hygiene passed.
• Residual note: cargo fmt --check reports broad pre-existing repository drift outside this focused change.
• Dependency: this helper requires the API deployment containing datastream PR #1363.