Add ThreeMfDef FileDef subclass for 3MF (3D printing) files - #5628

Background and Goal

Adds ThreeMfDef, a FileDef subclass for .3mf (3D Manufacturing Format / 3D printing) files, so they index as a first-class file type. A .3mf is an OPC package (a ZIP) whose 3D/3dmodel.model XML carries title / designer / description / license and a unit — searchable text that is not present in the filename — so the subclass extracts it and feeds it into full-text search. Distinct icon + light metadata templates; no 3D viewer.

Resolves CS-12054.

Where to start

• packages/base/3mf-meta-extractor.ts — the parse. fflate.unzipSync with a filter that records every entry name (→ hasThumbnail) while inflating only the *.model part; then decodes ≤256 KB of the model, cuts at <resources>/<build>, and regex-extracts the metadata + unit. Throws FileContentMismatchError on a non-ZIP or a ZIP with no model part.
• packages/base/3mf-file-def.gts — ThreeMfDef extends FileDef: the six searchable fields, extractAttributes (super + extract3mfMetadata), cube-3d-sphere icon, acceptTypes, and metadata-display isolated/embedded templates.
• packages/runtime-common/file-def-code-ref.ts — the one-line registry entry that maps .3mf → ThreeMfDef and lights up urlNamesFile / isFileDefCodeRef.

Key decisions and non-obvious mechanics

• Bounded on the indexing hot path. extractAttributes runs for every file during the index/prerender visit and 3MF files can be large, so we inflate only the model part and cap the XML parse at the header (metadata lives above <resources>; geometry lives below). Counting objects/triangles or reading the bounding box would require scanning past all vertices, so those are deferred.
• Graceful fallback. A malformed/non-3MF file throws FileContentMismatchError, so the extractor falls back to a bare FileDef and the file still indexes — same pattern as Mp3Def/PngDef.
• New dependency: fflate (~8 KB, isomorphic, added to the catalog + base). No ZIP/deflate library existed in the repo; base already imports npm runtime deps this way (super-fast-md5, awesome-phonenumber). Also added to host so the acceptance test can build fixtures.
• MIME + binary. .3mf → model/3mf (IANA-registered; mime-db has no entry) and classified binary, so the CLI/bot don't treat the ZIP as text.
• Deferred to follow-ups: a WebGL/three.js viewer (greenfield + prerender-render risk), the expensive geometry fields (triangle/vertex counts, bounding box, volume, object count), and surfacing the embedded thumbnail as the preview image.