Add path filtering and explain metadata to search APIs

[fa1k3]

Summary

Adds folder-scoped search filtering and richer score explanation metadata across qmd search APIs.

Changes

• Add pathPrefix filtering to SDK/MCP/REST search paths and --path/-p to the CLI.
• Preserve original indexed file paths in documents.original_path and use them for user-facing path filtering/results, while keeping existing normalized path compatibility.
• Preserve original paths in document lookup, CLI get, and CLI/SDK multi-get as well, so a path returned by search (for example qmd://collection/+Projects/Some Folder/file.md) can be passed back to qmd get, MCP/SDK lookup, and glob multi-get without manually converting to the normalized slug form.
• Make pathPrefix folder-only, case-sensitive, boundary-safe, and slash-normalized:
  • stack matches stack/...
  • stack does not match stack0/... or stack-monitor/...
  • Stack and stack remain distinct
  • empty string and / behave as no filter
• Add explain.scoreType and per-result explain.backendSources so callers can distinguish RRF-position scores from rerank-blend scores and see whether a result came from BM25 and/or vector evidence.
• Harden HTTP /query JSON handling for malformed JSON and invalid pathPrefix / explain types.

Semantics

pathPrefix is intentionally folder-scoped, not file-prefix matching. File-like prefixes such as README.md normalize to a folder prefix and therefore do not match the file itself.

Tests

Latest targeted verification after the original-path roundtrip fix:

• bun test test/path-filter.test.ts test/score-breakdown.test.ts test/cli.test.ts --timeout 60000 --preload ./src/test-preload.ts — 137 passed
• npm run test:node -- test/path-filter.test.ts test/score-breakdown.test.ts test/cli.test.ts — 137 passed
• bun run build
• npm run test:types

Earlier full-suite verification on this PR branch:

• npm run test:node -- test/path-filter.test.ts test/score-breakdown.test.ts test/mcp.test.ts — 70 passed
• npm run test:node — 904 passed
• npm run test:bun — 904 passed