Document supported pseudocode with section 7 language manuals

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

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+% CPSC-FUNCTION-IS_NUM(7)
+
+# NAME
+
+cpsc-function-is_num - test whether text represents a number
+
+# SYNOPSIS
+
+```text
+IS_NUM(text : STRING) RETURNS BOOLEAN
+```
+
+# DESCRIPTION
+
+Return true for a nonempty string fully consumed by the runtime's
+numeric conversion. Leading whitespace is accepted; trailing whitespace
+or other trailing text is rejected.
+
+# EXAMPLE
+
+```text
+OUTPUT IS_NUM("12.5")
+```
+
+# SEE ALSO
+
+[cpsc-language-overview(7)](cpsc-language-overview.7.md), [cpsc-function-str_to_num(7)](cpsc-function-str_to_num.7.md).
+