feat(sdk): expose placement presentation validators

[bokelley]

Closes #2800.

Summary

• expose placement presentation validators from the package root and their types from supported entrypoints
• document SSRF-safe, digest-pinned placement presentation validation
• add CJS, ESM, and declaration-condition package export smoke coverage

Validation

• env -u DATABASE_URL npm run ci:quick (16,971 fast tests, 413 slow tests, 30 ESLint-plugin tests, 18 renderer tests)
• npm run ci:docs-check
• three independent expert reviews with no remaining findings

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Open workspace in Conductor

[aao-secretariat]

Ladon verdict: Approve

Approve — clean additive PR.

This PR exposes placement presentation validators (PlacementPresentationDocumentSchema/ReferenceSchema and their types) at the package root plus /schemas and /types, with accurate SSRF- and digest-pinned validation docs and thorough CJS/ESM/declaration-condition export smoke tests. All exports resolve to real generated symbols with verified same-instance identity; the witness-not-translator invariant and schemas-only policy guard both hold.

No blocking or medium findings. One Low nit only (changeset is patch but new public exports arguably warrant minor) — Low findings are omitted from the gate and do not block.

Decision-table walk: no critical/high findings (row 1 n/a); gated_paths false (row 2 n/a); high_risk false, no deletions (rows 3/5 n/a); no medium findings (rows 4/8 n/a); no no-auto-approve team match (row 7 n/a); no prior decision (row 6 n/a). Falls through to row 9 → approve.

Note: review_decision is REVIEW_REQUIRED, but since gated_paths is false this does not force escalation.