Add a reusable page geometry layer on top of native text pagination so callers can query page-relative rectangles and map rectangles back to text ranges without breaking offset continuity.
<must_haves>
RDEPUBTextLayoutFrame can expose page geometry derived from CoreText, not guessed from offsets.
pageStartOffset, pageEndOffset, fragmentOffsets, and RDEPUBTextOffsetRangeInfo semantics remain stable.
The geometry API is reusable by selection, search, highlight, and tap-locate work in later plans.
</must_haves>
execute
Extend `RDEPUBTextLayoutFrame` and the page model with explicit geometry value types, including a page-level geometry container and line or fragment geometry records. Populate them from `RDEPUBTextLayouter` using CoreText line data so the page can answer range-to-rect and rect-to-range queries without changing pagination break rules.
- Sources/RDReaderView/EPUBTextRendering/RDEPUBTextLayoutFrame.swift
- Sources/RDReaderView/EPUBTextRendering/RDEPUBTextLayouter.swift
- Sources/RDReaderView/EPUBTextRendering/RDEPUBTextPaginationSupport.swift
- Sources/RDReaderView/EPUBCore/RDEPUBReadingModels.swift
- `RDEPUBTextLayoutFrame` or its companion geometry type exposes a query path for at least string-range rectangles and reverse lookup from rects to `NSRange`.
- Existing metadata fields (`contentRange`, `breakReason`, `blockRange`, `attachmentRanges`, `attachmentKinds`, `trailingFragmentID`, `diagnostics`) remain present and unchanged in meaning.
- `RDEPUBTextLayouter` still emits the same page boundaries for the sample books after the geometry data is added.
execute
Thread the new geometry data through `RDEPUBTextBookBuilder` and `RDEPUBTextBook` so each page retains its geometry alongside `pageStartOffset`, `pageEndOffset`, and `fragmentOffsets`. Keep `pageNumber(for:)` and `location(forPageNumber:)` compatible with the existing absolute-offset contract.
- Sources/RDReaderView/EPUBTextRendering/RDEPUBTextBookBuilder.swift
- Sources/RDReaderView/EPUBCore/RDEPUBReadingModels.swift
- Sources/RDReaderView/EPUBTextRendering/RDEPUBTextLayouter.swift
- Sources/RDReaderView/LegacyRDReaderController/RDEPUBTextPaging.swift
- `RDEPUBTextBook` pages still map absolute offsets to pages through `pageStartOffset` / `pageEndOffset`.
- `RDEPUBTextBook.location(forPageNumber:)` still returns a stable `RDEPUBLocation` for native text pages.
- The sample text book builds successfully with the new geometry fields attached to each page.
- Build the `ReadViewDemo` workspace with the `ReadViewDemo` scheme.
- Confirm the new page geometry API compiles and the sample book still paginates.
- Confirm no existing offset-based selection or restore code needs to change to keep building.
<success_criteria>
Native text pages expose reusable geometry data.
The offset-based page contract remains intact.
Later selection overlay work can consume the geometry layer without re-deriving page boundaries.
</success_criteria>
