fix(pptx): carry verbatim custGeom OOXML in deck JSON (cross-process serialize)

[karthikmudunuri]

Path B from the eon-deck custGeom investigation — the architecturally-correct fix for the cross-process failure mode (parse client-side → store deck JSON → serialize server-side).

Root cause (confirmed in code)

High-fidelity replay of imported elements depends on two module-global registries — sourceBufferCache (pptxToDeck.ts:366) and elementSourceRegistry (line 394) — populated only by parsePptx and never serialized into the deck JSON. Per-element verbatim replay gates on getElementSource(el.id) (deckToPptx.ts). So when serialize runs in a different process from the import, the registry is empty → every element is re-synthesised from its deck fields.

Synthesis can't represent OOXML even-odd / winding (custGeom has no winding flag), so complex vectors blank. I verified the bicycle's synthesized custGeom is structurally perfect (6 closed subpaths, coords fill the box, no NaN) — which is exactly why the fix isn't "patch the synth" but "don't synth at all when we have the source."

Fix

The importer stamps the verbatim <p:sp> of self-contained custGeom shapes onto the element as pristineOoxml = { xml, snapshot }, which rides in the deck JSON. On serialize, an unedited such shape (snapshot still matches) is replayed verbatim — exact source winding/geometry — instead of re-synthesised; cNvPr/@id is rewritten to avoid spTree collisions. Edited shapes fall back to synthesis.

Same persist-in-JSON pattern already used for embedded fonts (deck.fonts), scoped to vector shapes so JSON bloat is negligible (a few KB/deck — the collaborator measured ~13 KB across this 25-slide deck).

Self-contained only: shapes whose source XML references r:embed/r:id/r:link (images) or a:schemeClr (theme colours) are not stamped — the fragment would be invalid without the source archive/theme. Those still synth.

Coexists cleanly with same-process replay: in-process, the shape is caught earlier by isPristineImportedElement (registry hit) and replayed through the source archive; pristineOoxml is the cross-process fallback only.

Tests (+3, 61 pass)

• Importer stamps pristineOoxml (with custGeom + snapshot) on a round-tripped self-contained custGeom shape.
• Serialize replays the verbatim <p:sp> (source fill + marker present) and rewrites the colliding cNvPr id.
• Edited shape (snapshot diverges) falls back to synthesis.

tsc -b clean, build:lib succeeds. Branch is off post-#68 main.

Scope note

This is the durable class-killer for cross-process custGeom (and the pattern extends to radial/other preserved elements later). The fill="darken" + path→EMU synth fixes from #66 are already merged; this is complementary — it makes unedited vectors byte-faithful regardless of process.