documents.js: package-rebuild fidelity residuals -- embedded font bytes and wrap re-derivation through embedded faces

[Mearman]

Two residuals of the rebuild-from-package path, both structural properties of what a bare DocumentTree records:

• it carries no source-embedded font bytes, so a rebuild renders through pdf-codec's vendored substitutes and the standard 14 rather than the source document's own embedded faces (the one layer a package cannot carry)
• wrap re-derivation reproduces the original split wherever the original also resolved through the vendored/standard layers; where the source used an embedded face, the re-derived wrap and the re-render stay consistent with each other (both measure and draw through the same registry) but can differ from the original's actual layout

Evidence

#964's README wording; src/convert/from-package.ts module comment.

Affected packages: documents.js, pdf-codec