feat: public ModelPicker sample over ModelSelection + attachImage/togglePin gap closers

[roryford]

PR 3 of the reusable model-selection plan (docs/plans/reusable-model-selection-surfaces.md). Builds on PR 0 (#1866), PR 1 (#1864), PR 2 (#1873). Draft — do not merge.

1. Public ModelPicker sample over ModelSelection

• Promotes the private ModelSelectionTabView → public ModelPicker in ManifoldUIModelManagement: a thin, styleable sample selector. The headless ModelSelection stays the product; ModelPicker is the default, not the only path.
• Adds a grouping seam (grouped: Bool) that renders the foundation-vs-downloaded sections from ModelSelection.groupModels(_:by:). Defaults to false (the historical flat list).
• Hoists the grouping into a static ModelSelection.groupModels(_:by:) so the sample view and the headless instance method share one implementation — no ModelLoadCoordinator needed to render. ModelPicker drives selection through a ModelRegistry it already owns (no service widening; respects the service-sharing rule).

2. ModelManagementSheet composes ModelPicker

• Re-expressed to use ModelPicker(grouped: false) in the Select tab — no behavior change. Existing logic/snapshot/explicit-init tests updated to the new name.

3. Gap closers

• ChatViewModel.attachImage(_:mimeType:) ([API] Extend ModelCapabilities with codeGeneration and multilingual axes #1298) — convenience over stageAttachment that builds the MessagePart.image and routes through the same internal path the bundled composer uses (placeholder-hash parity verified).
• SessionManagerViewModel.togglePin(_:) ([API] Bulk session deletion on SessionManagerViewModel #1300) — pins/unpins based on the record's isPinned.

4. Docs snippet-gate fix (Correction G / Decision 6)

• Adds ManifoldUIModelManagement as a third linked product in scripts/extract-snippets-test.sh (it is not umbrella-re-exported) so ModelPicker doc snippets compile.
• New ManifoldUIModelManagement.docc/ModelPickerSample.md article with three compile-checked ModelPicker snippets (not no-build, not under a blocked heading). Links into the existing docs/QUICKSTART-MODEL-SELECTION.md (owned by PR 2) via prose rather than editing it.

Validation (local)

• swift build --build-tests — clean.
• New slices green (ModelPickerTests, attachImage, togglePin); sabotage-checked then reverted.
• Snippet gate: extract-snippets.sh + extract-snippets-test.sh → 16 passed, 0 failed, including all three ModelPicker snippets, under the new 3-product scaffold. check-readme.sh passes.
• API digester clean on ManifoldInference / ManifoldUI / ManifoldUIModelManagement — all additions are additive (no allowlist entry needed).

Resolves #1298
Resolves #1300

🤖 Generated with Claude Code

[roryford]

Orchestrator review (adversarial): approve. groupModels hoist is a clean single-implementation refactor (instance groupedModels forwards to the new static), and the sample view correctly drives selection through its own ModelRegistry rather than the service coordinator — no service-sharing-rule violation. attachImage delegates to the same internal stageDraftAttachment path as ChatInputBar (not a parallel path); togglePin reads isPinned at call time over the existing idempotent pin/unpin. Snapshot test is a mechanical view-rename, not a masked regression. Digester clean (all additive). Disjoint from #1874/#1875 — merging on green.