docs: remove the seven passages three upstream fixes made false (Phase G)

[ocots]

Summary

Three upstream defects were documented on this site as known, cited issues:

• CTParser#338 — a misleading
  Line n: prefix on parser errors
• CTSolvers#216 /
  #222 — the GPU
  ExtensionError names the wrong missing package
• CTDirect#624 — :variable
  advertised but not implemented

All three are fixed upstream (CTParser ≥0.9.1-beta, CTSolvers ≥0.5.5-beta, CTDirect
≥1.1.2-beta), and a fresh resolve confirms this package's current [compat] already picks
versions carrying every fix. The seven passages describing these as live bugs now state the
opposite of what the code does, or document a scheme that no longer exists.

What changed

docs/src/solve/choosing-a-method.md

• Drop the "with one exception" clause on the ExtensionError paragraph (CTSolvers#222 fixed
  the predicate — there's no exception left).
• Delete the :variable scheme table row and the ":variable is advertised but does not run"
  box entirely — the scheme was removed upstream (CTDirect 1.1.2-beta), not fixed, so the
  box's premise is false.
• Delete the trailing paragraph citing CTParser#338 inside the "Under :exa" box, keeping its
  still-accurate prose and the live @repl block.
• "Two of the results need spelling out" → "One", now that only the :exa box survives.

docs/src/solve/gpu.md / docs/src/getting-started/installation.md

• Rewrite the "it points the wrong way" paragraphs — the ExtensionError now correctly names
  CUDSS when that's the package actually missing.

docs/Project.toml + docs/src/assets/Project.toml

• Drop the self-referential OptimalControl [deps] entry (no matching [compat] line
  existed for it). docs/make.jl loads this package via LOAD_PATH stacking
  (--project=. from the repo root — see the file's own header comment), never through
  docs/Project.toml.

Verification

• julia --project=. docs/make.jl: clean build, both before and after the Project.toml
  edit. Unresolved @ref count unchanged at 6.
• grep -rn "CTParser#338\|CTSolvers#216\|CTSolvers#222\|CTDirect#624\|Line 6:" docs/src →
  no matches.
• Grepped the whole docs/src/ tree beforehand for any other reference to these four issues,
  or any @ref/anchor pointing into the deleted boxes — none found.
• docs/src/assets/Manifest.toml deliberately left untouched: rebuilding picked up unrelated
  dependency-version drift (CTDirect, CTSolvers, CTParser betas that moved since this snapshot
  was last generated) that doesn't belong in this PR.

No [compat] change, no source change — pure documentation coherence, independent of the
compat-raise phase blocked on CTDirect#629.

🤖 Generated with Claude Code