forms, initials: walk the field and page trees with the shared walker

[vanbroup]

Summary

Follow-up to #169/#170, which moved signing and verification onto the shared AcroForm walker in internal/acroform. Two places outside that path still walked /Kids with loops of their own, without a visited set, a depth bound, or /FT inheritance:

• forms.Extract / forms.MapFields / Document.applyPendingFields walked the field tree by recursion. A field whose /FT sits on its parent (inheritable, ISO 32000-1 Table 220) was left out, and a crafted /Kids cycle recursed without end.
• initials placement (findPage) recursed into the page tree /Kids the same way.

Changes

• internal/acroform gains a general Fields(root, fn) and FieldsOf(field, prefix, fn), reporting every field (a parent before its kids) as a Field{Dict, Name, Type, Value, Terminal} with the inherited /FT and /V resolved and the fully qualified name built from /T decoded as a text string. SignatureFields is built on the same walk with the one signature-specific rule kept: a signature field that carries its own /V is the signed field and is not walked into.
• Per ISO 32000-1 12.7.3.1 a terminal field is one without field kids, and a parent's /V is the default of its kids (Table 220). forms.Extract now reports terminal fields with their inherited type and value; MapFields maps every typed field as before, and the document's pending field updates are resolved with the same walk.
• A field's name is its partial names joined with periods, so an untitled intermediate node no longer adds an empty component.
• The visited set is keyed on the object pointer and the effective type: a conforming field has one parent, but a crafted tree can hold the same object under parents passing down different types. Keyed on the pointer alone, the first path decided, and a signed field reached first under a text field was skipped under its signature parent. It is now walked once per type, while a cycle still ends.
• findPage carries a visited set and a depth bound, seeds the root node as visited, and returns an error when the page tree holds no such page instead of a null value the caller would write an annotation against.
• Test fixtures share one PDF builder in internal/testpdf.

Related

This PR is independent of #171 and #173, which fix the verifying path; it can merge in any order with them.

Reviews

Code review and security review were run on the branch; the code review findings (generalized "/V stops descent" rule breaking forms; missing-page error) are fixed, and the security review raised no findings beyond the shared-node dedupe note addressed above.

Test plan

• go vet ./... && go test ./... (12 packages green)
• internal/acroform: inherited /FT and /V, terminal flag, untitled intermediate node, UTF-16 /T, self-reference and cross-parent cycles, shared node under parents of different types
• forms: nested fields named in full with inherited type, parent /V as default, /Kids cycle terminates, MapFields maps typed parents and terminals
• page tree: cycle terminates, missing page reported as an error
• extract and verify field tree tests moved onto the shared fixture builder