Expose `basePath` on `CompileOptions` to match `LintOptions`

[dean0x]

Location: packages/mds/src (LintOptions vs CompileOptions)

API asymmetry: LintOptions exposes basePath, but CompileOptions does not — even though the underlying napi compile already supports it. Consumers can scope lint resolution to a base path but cannot do the same for compile, despite the capability existing in the native layer.

Deferred From: PR #171 (Closes #61) — mds lint release gate — cycle-2 review (I-15)
Review: .devflow/docs/reviews/feat-mds-lint-61/2026-07-12_0020/resolution-summary.md

Reason for Deferral:
The fix touches the compile public API surface, which is unrelated to the lint branch's purpose. It should land as its own focused API change with matching tests/docs rather than being smuggled into the lint release gate.

Acceptance Criteria:

• CompileOptions exposes basePath with the same semantics/typing as LintOptions.basePath
• The option is threaded through to the napi compile call (which already accepts it)
• Type surface stays consistent across packages/mds/src (types.ts, index, node, backend); tsc --noEmit -p packages/mds clean
• Tests cover compile honoring basePath; npm test -w @mdscript/mds passes