Add XNEdit workload with Xft text input

[jserv]

Bring up XNEdit (Motif NEdit fork) as a libx11-compat workload exercising Xft/fontconfig text rendering, and fix the interactive keyboard and Xft paths it surfaced.

Keyboard/input:

• XKeysymToKeycode round-trips through XkbKeycodeToKeysym so SDL scancode keys (e.g. SDLK_EXECUTE) no longer alias onto ASCII keycodes; resolves shifted punctuation to its base key.
• Single usShiftPairs[] table drives shiftedKeysym/unshiftedPunctuation, feeding XkbLookupKeySym and XkbKeysymToModifiers for US-layout symbols.
• Xutf8LookupString guards NULL events and reports overflow lengths; appendKeysymUtf8 encodes ASCII/Latin-1/Unicode keysyms to UTF-8.
• XTestFakeKeyEvent threads held modifiers.

Xft/font:

• Extend XftDrawSetClipRectangles, FcCharSet/FcObjectSet overflow guards.
• normalizeGlyphAlpha rescales blended glyph alpha for legible text.

---

Summary by cubic

Adds XNEdit as a Motif workload via libx11-compat with libXt-compat and SDL_ttf‑backed libXft-compat, validating Unicode text rendering through Xft/fontconfig. Wires XNEdit into the differential CI and fixes keyboard, Xft/drawing, color, and build issues surfaced by the editor.

• New Features

  • Build XNEdit via mk/xnedit.mk; add docs, screenshot, and smoke/differential UI replays (startup, UTF‑8 fixture, typing).
  • Add scripts/run-xnedit-differential-tests.py; include XNEdit in the GitHub differential workflow; map replay keycodes to xdotool; screenshot capture falls back to the renderer when needed.

• Bug Fixes

  • Keyboard/input: round‑trip KeySym↔KeyCode via XkbLookupKeySym; single US Shift‑pairs table; Xutf8LookupString overflow reporting and UTF‑8 encoding for ASCII/Latin‑1/Unicode; XTestFakeKeyEvent threads held modifiers; libXt-compat patch sets XInput focus; release active grabs when windows become unviewable.
  • Xft/font/drawing: implement XftDrawSetClipRectangles; safer fontconfig stubs (FcCharSet, FcObjectSet, FcUtf8ToUcs4, listing/match) with overflow guards; normalize blended glyph alpha; codepoint‑aware font fallback; mixed‑charset open falls back to a base font when host fonts are missing; add a text‑stamp cache and present path to avoid non‑idempotent re‑blends, flushing stamps on unmap/reparent.
  • Colors/display: support X11 rgb: color parsing; optional LIBX11_COMPAT_SCREEN_GEOMETRY override.
  • Build/CI: fix libXt patch staging parallel‑build race; silence benign SDL3 typedef redefinition; clear MAKEFLAGS/MFLAGS for the upstream system XNEdit build in differential runs.

^{Written for commit 888a076. Summary will update on new commits.}