feat: provider selection and document OCR UX

[forhappy]

Summary

This PR consolidates the remaining unmerged provider-selection and document/OCR work.

Included

• Explicit semantic LLM provider/model selection with non-secret environment defaults, CLI precedence, actionable diagnostics, and provider configuration guidance.
• OCR/document processing hardening for PDF, DOCX, PPTX, and XLSX workflows, including bounded provenance, OCR mode/profile metadata, cache-safe projection, and managed model status/install UX.
• A document viewer experience with Combined, Native, and OCR evidence tabs; confidence badges; page/slide/sheet locators; click-to-preview OCR geometry; partial-coverage warnings; and strict bounded graph contracts.
• Deterministic viewer assets, VS Code initialization onboarding for managed OCR, compatibility documentation, and regression coverage.

Framework graph work is intentionally out of scope for this PR: React, Next, React Router, TanStack, Vite, universal framework evidence/resolution, conservative ambiguity handling, render relations, route stages, client/server boundaries, file-set matching, framework fixtures, qualification scripts, and framework-specific VS Code/task-context plumbing are excluded.

Generated local .vercel state and screenshot artifacts are excluded.

Verification

• CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-session-all-pr cargo check --workspace --all-targets --locked
• CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-session-all-pr cargo test -p compass-core --lib --locked (80 passed)
• CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-session-all-pr cargo test -p compass-output --lib --locked (57 passed)
• CARGO_TARGET_DIR=/Volumes/Workspace/crabbuild-target/compass-session-all-pr cargo test -p compass-files --lib --locked (13 passed)
• Provider CLI regression and dedup diagnostic tests (passed)
• npm run typecheck:js
• npm run test:js (42 viewer files/215 tests, 43 VS Code files/141 tests, 88 browser tests)
• node scripts/check_viewer_assets.mjs
• sh scripts/check_product_boundary.sh

cargo fmt --all -- --check still reports the pre-existing one-line cfg formatting drift in crates/compass-ocr/src/engine.rs; the focused Clippy command is also blocked by the pre-existing assert!(false) test assertion in crates/compass-core/src/task_context.rs.