Always set every style data field so a stale value cannot be stranded

[kmcginnes]

Description

A node could keep displaying a stale icon forever, with no error.

cy.json({ elements }) merges element data: ele.data(obj) adds and overwrites keys but never deletes ones missing from the new object. Both of the optional style data fields were therefore unclearable once applied. Concrete path: a user replaces a type's icon, the new icon fails to load, iconRegistry retries three times and gives up, the type drops out of the background-image map, __iconUrl goes absent — and the node silently keeps rendering the previous icon while its colour updates around it. The user gets no signal the upload failed and reasonably concludes it worked.

This is a regression from the data() migration in #2112, not preserved behaviour: the old stylesheet path set background-image unconditionally, so undefined cleared it.

The fix is to make the fields total rather than to gate on presence. ge_iconUrl carries "none" (cytoscape's no-image value) when a type has no icon, and ge_lineDashPattern carries cytoscape's default [6, 3] for solid lines, which ignore it. Both then live on the base node / edge rule and always reflect current state, so there are no gated selectors left — the stylesheet is two rules.

Note that adding background-image: none to the base rule alone would not have fixed this: the gated selector keeps matching while the key is still present, and overrides it.

Two side benefits: the icon gate moves out of the generic components/Graph component next to the producer that feeds it, and __iconUrl is renamed ge_iconUrl to match its siblings. ge_lineDashPattern had the same defect latently — harmless today because cytoscape ignores the pattern for solid lines, but the same shape.

How to read

1. graphElementStyleData.ts — the ALWAYS_SET rationale and the two now-total fields.
2. useGraphStyles.ts — mappers moved onto the base rules, gated selector deleted.
3. useManageStyles.ts — the hardcoded gate deleted from the generic component.
4. The ADR gains the merge-semantics constraint, since it is the reason for the design.

[kmcginnes]

Superseded by #2128, which consolidates this stack into a single PR targeting #2112. Same commits, same end state — the branch there is byte-identical to the tree these were cut from, rebased onto schema-view-style-perf after #2118 was squash-merged into it.