docs: v0.1.0 alpha framing + hygiene pass

Update the public-facing docs to the M-49 state (v0.1.0 alpha, first
full-campaign playtest: 19/21 missions completable, D191 crashes Bunker ii
+ Statue, D193 AI pacing is the top flaw).

- README: lead with what it is + status, research angle second; new
  Quick start section; Status/Download rewritten for the alpha + dual
  platform; "By the numbers" refreshed (~3 weeks, ~320 commits, ~190
  findings, ~49 sessions, 69 PORT files).
- .github/release-notes.md: v0.1.0 title + known-issues rewrite.
- .github/social-preview.html: status/stat lines (regen + re-upload the
  PNG separately).
- docs/index.md, docs/dev/LEVEL-STATUS.md: M-49 status.
- docs/dev/findings.md, docs/dev/GRAPHICS-BACKLOG.md: D191-D197 +
  D148/D170/D173 M-49 escalations.

Hygiene:
- Fix the `&` literal in internals.md H1.
- Repoint two ../README.md links that 404 on the Pages site.
- Standardise British -> American spelling across the public docs
  (behaviour->behavior, colour->color, catalogue->catalog,
  centre->center, etc.). Deep internal logs left untouched.
- .gitignore: document the local-notes convention (docs/HANDOFF.md,
  docs/BACKLOG.md, and a new docs/dev/notes/ drop for roadmaps and
  session plans) so it survives a fresh clone.

docs/dev/LEVEL-STATUS.md

@@ -7,6 +7,16 @@ auto-injected, D121), `GE_PCDUMP="80-260:40"`, ~24 s watchdog then
 0x140000000). "render %" = `tools_pc/pixcount.py` non-clear on the last
 captured frame. Binary at `f2beae4b` + this session's build.
 
+**M-49 (2026-09-04) — full-campaign playtest, v0.1.0 win64 bundle.**
+User played every solo mission on the packaged build (fresh save). **19 / 21
+completable start to finish.** Only two crashes in the whole game — **Bunker ii
+(`-level_27`) and Statue (`-level_22`)**, same fault PC `0x1400801b1`
+(`modelGetNodeRwData` node-tree walk), one root cause = **D191** (pointer-width
+/ byte-swap family, findings §F). This retires the old "expect crashes past the
+level intro" framing for 19/21 levels. Biggest non-crash complaint: **D193** —
+AI locomotion too slow, breaks Cradle's scripted progression. v0.1.0 tagged off
+this state.
+
 **M-30 re-sweep (2026-08-31, build after D157/D120/RC2): 21 / 21 PASS — no
 crash logs.** First all-green sweep incl. Cuba (54) — the D129 credits-path
 crash no longer reproduces on a bare `-level_54` boot (Cuba rendered 92.5%