M6: a saved Password field returns to its default value and is redrawn from it (#3926)

Review on #3943. A black-box test of Acrobat (synthetic form, Save As and
Ctrl+S, every decoded object searched) showed how it saves a Password
field: the typed value is nowhere in the file; a field with /DV is saved
holding /DV, one without is left empty; the asterisks are drawn from the
saved value (5 for a 5-letter default, not 13 for the typed password);
/DV and /AA are kept.

clear_password_values now does the same: it restores the (inherited)
/DV as the value, or leaves the field empty, and redraws the widgets, so
the appearance no longer shows how long the typed password was. A field
already at its default no longer counts as holding a password, so such
a form needs no cleaned copy. The parity note for form.fill-text says so
and lists the two limits Acrobat shares (a script copying the password
into an ordinary field, and an earlier revision kept by an incremental
save).

Mutants, each failing a test: no /DV restore, no redraw, the default
ignored in password_fields, emptiness ignored (an empty string in an
array). Dropping the empty-default filter was equivalent, so the filter
is gone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

parity/acrobat-features.toml

@@ -4440,8 +4440,8 @@ milestone = "M6"
 status = "shipped"
 commands = ["form.fields"]
 tools = ["form_fill", "form_fields"]
-evidence = ["crates/ui-egui/tests/it/forms.rs::typing_into_a_text_field_and_tabbing_on", "crates/ui-egui/tests/it/forms.rs::auto_size_fields_shrink_and_wrap_typed_text_to_fit_their_box", "crates/forms/src/tests.rs::text_fields_get_new_appearances", "crates/engine/src/tests.rs::filling_a_form_shows_saves_and_undoes", "crates/automation/tests/it/automation.rs::orphan_form_fields_through_tools", "crates/forms/src/tests.rs::fields_sharing_a_name_take_the_same_value", "crates/ui-egui/tests/it/forms.rs::hovering_a_field_shows_its_tooltip", "crates/forms/src/tests.rs::zero_max_len_is_unlimited_and_overrides_inherited_limit"]
-notes = "in-place editor; Enter or clicking away commits, Escape cancels; auto-size (0 Tf) fields shrink, and multiline ones wrap, the text being typed to fit the box, on screen as on the page (the screen measures in the interface font, so its size can be a little smaller than the page's Helvetica fit); fixed sizes are kept and clip, as in Acrobat; no format/keystroke JavaScript yet (M6.4); resting the pointer on any field shows its tooltip (/TU), as Acrobat does (#3854); imported MaxLen 0 permits text input and overrides inherited limits; positive limits remain enforced."
+evidence = ["crates/ui-egui/tests/it/forms.rs::typing_into_a_text_field_and_tabbing_on", "crates/ui-egui/tests/it/forms.rs::auto_size_fields_shrink_and_wrap_typed_text_to_fit_their_box", "crates/forms/src/tests.rs::text_fields_get_new_appearances", "crates/engine/src/tests.rs::filling_a_form_shows_saves_and_undoes", "crates/automation/tests/it/automation.rs::orphan_form_fields_through_tools", "crates/forms/src/tests.rs::fields_sharing_a_name_take_the_same_value", "crates/ui-egui/tests/it/forms.rs::hovering_a_field_shows_its_tooltip", "crates/forms/src/tests.rs::zero_max_len_is_unlimited_and_overrides_inherited_limit", "crates/engine/src/tests.rs::password_field_values_are_kept_in_memory_but_written_to_no_file", "crates/forms/src/tests.rs::a_cleared_password_field_returns_to_its_default_value"]
+notes = "in-place editor; Enter or clicking away commits, Escape cancels; auto-size (0 Tf) fields shrink, and multiline ones wrap, the text being typed to fit the box, on screen as on the page (the screen measures in the interface font, so its size can be a little smaller than the page's Helvetica fit); fixed sizes are kept and clip, as in Acrobat; no format/keystroke JavaScript yet (M6.4); resting the pointer on any field shows its tooltip (/TU), as Acrobat does (#3854); imported MaxLen 0 permits text input and overrides inherited limits; positive limits remain enforced. A text field with the Password flag is saved as Acrobat saves it (#3926, black-box test): the typed value is written to no file (Save, Save As, Reduce File Size, recovery, Extract/Split/Combine, signing); the field gets back its default value (/DV) or is left empty, and its asterisks are redrawn from that value. /DV and the field's scripts stay. Known limits, which Acrobat shares: a script that copies the password into an ordinary field saves it there (Acrobat recalculates a Calculate copy on save, PdfCraft does not), and an incremental save leaves a password stored by an earlier revision in that revision."
 
 [[feature]]
 id = "form.fill-checkbox-radio"