Document SLP-0002 local shared ownership

[julia-script]

Summary

• add the completed SLP-0002 local shared ownership reference, including exact Shared<T> contracts, allocation, cloning, scoped access, reentrancy, cleanup, local affinity, cycles, and pressure evidence
• document the sealed SharedCore<T> intrinsic boundary and stable diagnostics
• retain the accepted SLP-0001 independent-execution direction while marking its implementation as in progress across the language reference
• cross-link the new rules from the ownership, execution, entry, runtime, generic-representation, and intrinsic reference pages

Verification

• pnpm typecheck
• pnpm test (28 workspace tasks)
• pnpm --filter @silk-effect/compiler exec vitest run test/SharedStdlib.test.ts test/LocalSharedPressure.test.ts --maxWorkers=1 (13 tests)
• parsed all 22 language Markdown files and validated all new local links and anchors
• independent spec, implementation, and documentation-integration audits; post-fix re-review found no remaining issues

Local check note

pnpm check was also attempted in the original working tree, but Biome stopped on unrelated unstaged whitespace changes in BootstrapEvaluation.ts and InstanceDiagnostics.ts. Those compiler changes are not included in this PR.