lint: W013 (builtin shadowed by definition) never fires inside unobserved: or a match arm

[Nitjsefnie]

Describe the bug

W013 (function definition shadows a builtin) is never reported inside an unobserved: block or a match arm. It shares a walker with W012 — check_builtin_shadow — which recurses only through IF/LOOP/FOR/FUNC/TRY/PROGRAM, so with AST_MATCH (src/lint.c:799) and AST_UNOBSERVED (src/lint.c:801) in the break-only group the nested define is never visited and the emission at src/lint.c:741 never fires.

Filed separately from W012 because the two are distinct diagnostics, but a single fix to that walker's recursion closes both.

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:

define range(a) as:
    return a

Silent — inside unobserved::

unobserved:
    define range(a) as:
        return a

Silent — inside a match arm:

match 1:
    case 1:
        define range(a) as:
            return a
    case _:
        x is 0

Expected behavior

warning[W013]: 'range' is a builtin — function definition shadows it in all three.

Actual behavior

$ eigenscript --lint w013_control.eigs
w013_control.eigs:1: warning[W013]: 'range' is a builtin — function definition shadows it
$ eigenscript --lint w013_unobs.eigs
w013_unobs.eigs: no issues found
$ eigenscript --lint w013_match.eigs
w013_match.eigs: no issues found

Environment

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