lint: W012 (builtin shadowed by assignment) never fires inside unobserved: or a match arm

[Nitjsefnie]

Describe the bug

W012 (assignment shadows a builtin) is never reported inside an unobserved: block or a match arm. check_builtin_shadow recurses only through IF/LOOP/FOR/FUNC/TRY/PROGRAM, and both AST_MATCH (src/lint.c:799) and AST_UNOBSERVED (src/lint.c:801) sit in its break-only group, so the assignment is never inspected and the emission at src/lint.c:737 never fires.

Note this is the same walker as W013, so the two share a root cause even though the symptoms differ.

Surfaced while doing #779, which made these switches enumerable; it neither caused nor fixed this. Line numbers resolved against main @ 23edbc25.

To reproduce

Control — reported correctly:

range is 5
print of range

Silent — inside unobserved::

unobserved:
    range is 5
print of 1

Silent — inside a match arm:

match 1:
    case 1:
        range is 5
    case _:
        x is 0
print of 1

Expected behavior

warning[W012]: 'range' is a builtin — assignment shadows it in all three.

Actual behavior

$ eigenscript --lint w012_control.eigs
w012_control.eigs:1: warning[W012]: 'range' is a builtin — assignment shadows it
$ eigenscript --lint w012_unobs.eigs
w012_unobs.eigs: no issues found
$ eigenscript --lint w012_match.eigs
w012_match.eigs: no issues found

Environment

• OS: Debian 13 (x86_64)
• GCC version: 14
• EigenScript version: 0.34.0 (main @ 23edbc25)