feat(llm-docs): add the platform module for bundled on-disk docs

[midleman]

Summary

Step 2a of the bundled on-disk docs work: a host-agnostic platform/positronDocs module that downloads, verifies, caches, and serves the slim LLM docs bundle so the assistant can read docs from disk instead of fetching positron.posit.co. All I/O goes through three injected interfaces (HTTP, file store, archive), so the state machine is unit-testable with no DI container and no node imports.

• positronDocsBundle.ts -- types, URL resolution, bundle.json and checksum-file parsing
• positronDocsIO.ts -- the three I/O interfaces plus the ILocalDocs result shape
• positronDocsValidate.ts -- zip-entry traversal guard and extracted-bundle validation
• positronDocsCache.ts -- the orchestrator: install, version convergence, single-flight, hard-failure throttling
• 85 Vitest cases; 95% branch coverage on the orchestrator

Nothing is wired up here -- no registration, no instantiation, unreachable at runtime until the next PR adds the extension-host service. That PR must not merge before this one.

Release Notes

New Features

• N/A

Bug Fixes

• N/A

Validation Steps

Unit tests added:

npx vitest run src/vs/platform/positronDocs/

Two things worth a reviewer's attention:

• _writeState and cleanup deletes deliberately swallow their errors. The disk failures that break them are the same ones that break a download, so throwing would withdraw a valid cached bundle and violate the governing rule that a valid cache is always served.
• Release builds fetch their exact version and fall back to the mutable alias until it publishes, re-checking every launch. That convergence check is intentionally never throttled; only hard failures are.

[github-actions]

E2E Tests 🚀
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