[FXC-5651] feat: add client-side OBB computation via DraftContext.compute_obb()

[benflexcompute]

Summary

• Add DraftContext.compute_obb() for client-side Oriented Bounding Box estimation from surface geometry via PCA
• Supports List[Surface], single Surface, and SurfaceSelector inputs
• OBBResult is a frozen dataclass — all fields are properties: center, axes, extents, axis_of_rotation, radius
• rotation_axis_hint is passed at compute_obb() call time and baked into the result
• TessellationFileLoader downloads UVF manifest + bin files with two-layer caching (L1 in-memory, L2 disk)
• CloudFileCache: general-purpose size-based LRU disk cache at ~/.flow360/cache/ with module-level singleton
• Output is dimensioned (unyt) using project length unit cached on Geometry during from_cloud()
• Non-Surface selectors rejected upfront with clear error message

User API

with fl.create_draft(new_run_from=geometry) as draft:
    obb = draft.compute_obb(draft.surfaces["wheel*"], rotation_axis_hint=[0, 0, 1])
    print(obb.center)            # [x, y, z] m
    print(obb.radius)            # float m
    print(obb.axis_of_rotation)  # [x, y, z]
    print(obb.axes)              # [[ax0], [ax1], [ax2]]
    print(obb.extents)           # [e0, e1, e2] m

Test plan

• Unit tests: CloudFileCache (get/put, LRU eviction, self-eviction protection, overwrite accounting, oversized skip, graceful degradation)
• Unit tests: UVF parser (indexed/unindexed geometry, LOD, error paths)
• Unit tests: OBB compute (cube, cylinder, rotation_axis_hint selection, all-properties API)
• E2E tests: full pipeline through DraftContext with mocked downloads
• E2E tests: SurfaceSelector, List[Surface], single Surface input routes
• E2E tests: non-Surface selector rejection
• E2E tests: unyt dimensioned output with length_unit
• Manual E2E: verified against POC reference results on car model (4 wheels, exact match)
• CI passes

🤖 Generated with Claude Code

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Note

Medium Risk
Adds new geometry-processing logic (UVF parsing + PCA) and a persistent on-disk cache for cloud downloads, which may impact correctness and disk usage if edge cases or eviction behavior are off.

Overview
Adds client-side DraftContext.compute_obb() to compute an oriented bounding box from selected surface entities by downloading UVF tessellation data, extracting vertices, and running a PCA-based OBB algorithm (with optional rotation_axis_hint and LOD override).

Introduces TessellationFileLoader plus UVF parsing utilities, backed by a new size-based LRU CloudFileCache (shared singleton) to persist downloaded manifest/bin files across sessions and avoid partial-resource eviction.

Wires geometry-root drafts to include the tessellation loader and cached project length unit (from Geometry.from_cloud) so OBB outputs can be returned with units; adds comprehensive unit and E2E tests plus fixture tessellation data.

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