Sketch nodes refactor, global dimension settings, OCCT 8 wasm build docs

[trailcode]

Summary

• Refactor sketch nodes behind a PIMPL (Sketch_nodes::m_impl) while preserving snap and axis-guide behavior.
• Add global Settings → Sketch → Dimensions controls (line width, arrows, color, text scale, label rendering, placement, show/hide) with live refresh via Length_dimension_style and Occt_view::refresh_all_length_dimensions().
• Default label rendering to Z-layer Topmost to fix washed-out dimension text against the grid.
• Fix permanent node annotation size applying correctly from settings.
• Add OCCT V8.0.0 wasm build automation (scripts/build-occt-v8-wasm.ps1, .cmd) and docs/building-occt.md (Windows prebuilts + Emscripten).

Closes #113

Changes

Area	Notes
sketch_nodes.*	PIMPL refactor (~400 lines reorganized)
geom.*, gui_settings.cpp, sketch.cpp, occt_view.*	Dimension style bundle + apply on create/refresh
gui.h / gui.cpp	Settings keys, length_dimension_style()
usage-settings.md, README.md	Document nested Dimensions section + OCCT doc link
docs/building-occt.md, scripts/build-occt-v8-wasm.*	OCCT 8 wasm download/build guide and script

Removed non-working flyout / extension-line dimension settings (OCCT limitations).

Test plan

• Open Settings → Sketch → Dimensions; change line width, arrow size, color, text scale, label rendering, placement, arrow style/orientation, Show sketch dimensions — confirm sketch dims and extrude preview update live.
• Confirm dimension labels are readable over the grid (Z-layer Topmost default).
• Sketch snap: dimension-tool hover, cross-sketch snap, add-node on edge interior.
• Permanent node annotation size slider affects node markers.
• Save/reload ezycad_settings.json; verify edge_dim_* keys persist.
• Desktop Release build passes.
• (Optional) scripts\build-occt-v8-wasm.cmd after emsdk_env; emcmake EzyCad with printed OpenCASCADE_DIR.

Notes for reviewers

• Wasm OCCT 8 build path is experimental; desktop still targets OCCT 7.9.1 prebuilts in README.
• Commits WIP / Debug code may contain interim work — worth a quick scan before merge.