docs(preprocessor): document switches and library regions

Add manuals and aliases for all five PP directives, replace the retired #LIB
page, and update compiler and shared-editor architecture notes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 45 insertions, 0 deletions

diff --git a/doc/language/keyword/cpsc-keyword-pp_ifdef.7.md b/doc/language/keyword/cpsc-keyword-pp_ifdef.7.md
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+% CPSC-KEYWORD-PP_IFDEF(7)
+
+# NAME
+
+cpsc-keyword-pp_ifdef, cpsc-keyword-pp_ifndef, cpsc-keyword-pp_endif - conditionally compile source regions
+
+# SYNOPSIS
+
+```text
+PP_IFDEF name
+    source lines
+PP_ENDIF
+
+PP_IFNDEF name
+    source lines
+PP_ENDIF
+```
+
+# DESCRIPTION
+
+PP_IFDEF, PP_IFNDEF and PP_ENDIF are compiler extensions processed before lexing and parsing. PP_IFDEF includes its region when the named switch is defined; PP_IFNDEF includes it when the switch is undefined. Both require exactly one switch name. Each must have a matching PP_ENDIF, which takes no arguments. Conditions may nest, and a region is included only when all enclosing conditions are enabled. There is no PP_ELSE or condition-expression syntax.
+
+Directives occupy their own lines, optionally preceded by whitespace and followed by // comments. Keywords and switch names are case-sensitive. Switches belong to each file independently and change in source order through [cpsc-keyword-pp_define(7)](cpsc-keyword-pp_define.7.md).
+
+Disabled ordinary source is not lexed, parsed or compiled, and its PP_DEFINE and PP_UNDEF directives have no effect. Conditional nesting is still checked, including inside disabled regions. Directive lines and disabled source preserve original line numbering for diagnostics. Preprocessor conditions do not introduce ordinary scopes or add indentation in the VS Code plugin or Bundle.
+
+# EXAMPLE
+
+```text
+PP_DEFINE Debug
+PP_IFDEF Debug
+OUTPUT "Debug enabled"
+PP_IFNDEF Quiet
+OUTPUT "Verbose debug output"
+PP_ENDIF
+PP_ENDIF
+PP_UNDEF Debug
+PP_IFNDEF Debug
+OUTPUT "Debug disabled"
+PP_ENDIF
+```
+
+# SEE ALSO
+
+[cpsc-keyword-pp_define(7)](cpsc-keyword-pp_define.7.md), [cpsc-language-overview(7)](../cpsc-language-overview.7.md).