M4: custom document properties through the engine and the tools (#2076)

* M4: custom document properties through the engine and the tools (#organize.custom-properties)

/Info entries beyond the ones the spec names are custom properties:
organize lists them (custom_info) and set_info accepts any such name
after Acrobat's checks (no standard name, no empty name, no spaces,
delimiters, / or #, at most 127 bytes); inspection lists them too
(DocInfo::custom); doc_info reports them under custom and doc_set_info
sets or clears them. The Custom tab in Document Properties and the XMP
mirror stay planned.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz

* M4: custom document properties list the same before and after an edit

Review follow-up. The open-time reader (render/inspect.rs) now uses the
engine's rule (pdfcraft_organize::custom_info, trimmed, empty values
dropped): text strings only, NUL/whitespace trimmed, empty values left out,
and the 256 cap counting listed entries. Before, it also listed name
values (`/Dept /RnD`) and capped before filtering, so the list could change
after the first unrelated edit. An automation test opens a file with a name
value, blank, NUL-padded, empty and numeric entries and checks doc_info's
custom list is the same before and after setting the Title.

Drops the ROADMAP.md log line, which conflicted with main.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz

* M4: custom properties drop NUL/whitespace padding in one pass, before the cap

Review follow-up. custom_info now trims NULs and whitespace in any mix
(" \0Lab\0 " is "Lab"), leaves out values that are only padding, and does
both before taking 256, so blank entries can't crowd real ones out. It is
the one rule: the engine uses it as it is, and the open-time reader trims
the same way. Tests: 300 blank entries before a real one, and mixed
padding, in organize and through doc_info.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUNQSPVXx3dDBabRZHAnjz

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

apps/pdfcraft-cli/src/main.rs

@@ -329,6 +329,7 @@ fn info(args: &[String]) -> Result<(), CliError> {
         "bookmarks": info.outline.len(), "annotations": info.annotations.len(), "fields": info.fields.len(),
         "links": info.links.len(), "layers": info.layers.len(), "attachments": info.attachments.len(),
         "transitions": info.transitions.len(),
+        "custom_properties": info.custom.len(),
         "page_labels": info.pages.iter().take(8).map(|p| p.label.clone()).collect::<Vec<_>>(),
         "warnings": info.warnings,
     });