Add text on paths, OpenType controls, and paged GPU effects

[IAmJSD]

Text layers could not follow paths or expose OpenType features, and large warp/seam-carving jobs fell back to the CPU when a layer exceeded one GPU storage binding. This change addresses both README items.

• Add Type controls for kerning, ligatures, discretionary ligatures, small caps, and placement on a copy of the active path. Glyphs, carets, and selection follow the curve; settings survive PSD/PSB reopen and undo. Existing text retains its layout until feature overrides are used.
• Page warp sources into texture arrays and band their output. Add a paged seam-carving path with texture-backed image planes and a two-row cumulative-cost buffer, retaining the existing small-image path and device-limit fallbacks.
• Preserve fractional warp sampling at large document coordinates on CPU and GPU by keeping integer positions separate from interpolation weights. Add origin-invariance regressions while retaining the existing GPU parity tolerance.
• Give Type a compact single-row options bar with font, style, editable size, alignment, color, and commit/cancel controls. Put leading, tracking, OpenType toggles, and path settings in a docked Character panel. Numeric fields support replacement typing, negative values, and arrow-key adjustment without sending input to the text layer.
• Add regression and GPU parity checks and document the completed features.

Native CMYK/Lab editing remains unresolved in the README. docs/native-colour-editing.md explains why retaining independent native channels requires coordinated storage, undo, compositor, and plugin API changes. Text path copies do not track subsequent source-path edits; bidi paragraph layout and vertical writing remain unsupported.

Validation:

• make check-readme CARGO='cargo --locked': 185 tests, Clippy with warnings denied for the changed crates, and the app compile check.
• Desktop debug build and a visual check of the Type toolbar at 1280×900 completed in the original checkout before isolating this branch.
• Character UI: desktop debug build, app Clippy with warnings denied, workspace formatting, and a running-window check at 1280×900 covering numeric entry, negative values, arrow keys, feature toggles, and text commit.
• git diff --check.

This PR is based directly on main; the existing Schist Cloud branch and uncommitted gallery work are excluded.