lint: W010 (duplicate dict key) never fires inside unobserved: or a match arm

[Nitjsefnie]

Describe the bug

W010 (duplicate dict key) is never reported for a dict literal inside an unobserved: block or a match arm. check_dup_keys recurses through expressions and the IF/FUNC/RETURN/FOR/LOOP/PROGRAM/TRY/LIST node types, but AST_MATCH (src/lint.c:702) and AST_UNOBSERVED (src/lint.c:704) sit in its break-only group, so the literal is never reached and the emission at src/lint.c:631 never fires.

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:

w010_d is {"a": 1, "a": 2}
print of w010_d

Silent — inside unobserved::

unobserved:
    w010_d is {"a": 1, "a": 2}
print of w010_d

Silent — inside a match arm:

match 1:
    case 1:
        w010_d is {"a": 1, "a": 2}
    case _:
        x is 0
print of 1

Expected behavior

warning[W010]: duplicate dict key 'a' in all three.

Actual behavior

$ eigenscript --lint w010_control.eigs
w010_control.eigs:1: warning[W010]: duplicate dict key 'a'
$ eigenscript --lint w010_unobs.eigs
w010_unobs.eigs: no issues found
$ eigenscript --lint w010_match.eigs
w010_match.eigs: no issues found

Environment

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