pdf-codec: contextual GSUB, page-level widget annotations, and schema-gated image passthrough

[Mearman]

Three named follow-ups from the #967/#975 scorecards:

• GSUB applies liga/rlig through lookup types 1/4/7; calt/clig (contextual machinery out of proportion as a first cut) and smcp/dlig (opt-in in every real shaper) are refused by name, and nonzero-lookupFlag lookups are skipped because GDEF glyph classes are not read. The honest next step for contextual shaping is GDEF + chaining-contextual lookups.
• Form-field widgets ride the field's /Kids rather than their page's /Annots -- spec-legal, and this package's own reader recovers them symmetrically, but a viewer relying on page-level /Annots will not render them. pdf-codec's writer covers a small fraction of what its reader now extracts #967 flagged this as the follow-up it skipped.
• JBIG2/JP2 verbatim passthrough is blocked solely on the model carrying the original filter plus compressed bytes (tracked in the schema-gaps issue).

Evidence

#967 and #975 scorecards; src/gsub-table.ts refusal comment; src/write.ts form-field emission.

Affected packages: pdf-codec, document-schema.js