Components: Add renderer-owned text-fragment anchors and layout-generation tracking #4378

Goal

Provide a real native text-layout anchor path rather than approximating inline text with View geometry. Part of #4274; coordinate Popover #4236 and Tooltip #4249 consumers.

Remaining work and acceptance

• Review a renderer-owned UTF-16 text-fragment provider contract with attachment/surface/layout generations and explicit stale/unsupported outcomes.
• Support and qualify wrapped fragments, truncation, bidirectional text, embedded attachments, and layout changes; define which fragment(s) anchor placement uses.
• Implement actual Paper/Fabric providers for the declared hosts, preserving renderer-independent shared placement behavior. Public numeric tags, guessed global coordinates, and shadow-tree traversal are not substitutes.
• Test reflow, owner replacement/unmount, scroll/transform, stale layout generations, and cancellation/result races with native geometry evidence.
• Document host/version capability and keep unqualified paths out of root/all-host admission.

Prior art and evidence

The native-core milestone supports only materialized View and local rect/point anchors; no inline Text provider is qualified. Retired handoff: b55d6c9a3:packages/agentic/native-core/plans/modernization.md. Historical #3661 at 829883482ddd37324e64633126cc86ba08c2a75f contains Paper traversal, not a Fabric implementation; verify its current disposition before reuse. Candidate wave-three 5f6e7af1252bcfe087dcd1672afaa4ceb7c5efbb is reference evidence, not admission.