Compile: report a clear error when the readme block contains `*/`

[mazany]

Summary

A mod's readme lives inside a C block comment. If the markdown contains a
literal */, the comment closes early, the rest of the readme is parsed as
C++, and the resulting errors point somewhere else entirely. A targeted check —
"readme block contains a comment terminator at line N" — would turn a
mystifying failure into a one-line fix.

Why

The readme is markdown, and markdown in a mod's readme routinely contains code.
Any example that shows a CSS or C comment ends with */. So the trap is not
exotic: it is what happens the first time an author documents the very syntax
their mod reads.

The failure is unusually hard to read because the errors surface in a different
file. Nothing in the output points at the readme.

Evidence

Documenting a metadata header my mod parses out of CSS files, I wrote this in
the readme block:

    /* ==Patch==
    @name   Darker surface
    @theme  dark
    ==/Patch== */

The */ on the last line closed ==WindhawkModReadme=='s enclosing comment.
Compilation then failed like this (2.0.0-alpha.2, mod install --file):

electron-css-inject.wh.cpp:74:5: warning: '/*' within block comment [-Wcomment]
electron-css-inject.wh.cpp:79:1: error: expected unqualified-id
   79 | `@theme dark` or `@theme light` means the patch is only injected under that
electron-css-inject.wh.cpp:104:1: error: unknown type name 'with'
...
C:/Program Files/Windhawk/Compiler/include/windhawk_utils.h:381:9: error: use of undeclared identifier 'RemoveWindowSubclass'
C:/Program Files/Windhawk/Compiler/include/windhawk_utils.h:394:5: error: unknown type name 'SUBCLASSPROC'
...
4 warnings and 10 errors generated.

Ten errors, six of them inside windhawk_utils.h — a file I had not touched
and which is not the problem. The -Wcomment warning is the only real clue and
it is buried above errors that look far more serious. The obvious first theory
is a broken toolchain or a header incompatibility, which is a long way from the
truth.

Suggested shape

When splitting the source into metadata / readme / settings / code, scan the
readme block for */ and fail early:

error: readme block contains a comment terminator (*/) at line 76;
       it would close the enclosing comment. Escape it or reword the example.

The same check is worth applying to the settings block for the same reason.

Alternatives considered

• Let authors escape it. Needs a documented escape nobody will discover
  until after they've hit the bug — which is the bug.
• A different readme delimiter (fenced sentinel lines rather than a C
  comment) would remove the class of problem entirely, but that is a breaking
  format change for every existing mod. The diagnostic gets most of the value
  for a fraction of the cost and risk.

Tested on 2.0.0-alpha.2.