Feature: support --proxy-hints-with on faircode compare, not just profile

[yakew7]

Where: faircode/cli.py (compare subparser), faircode/SPEC.md section 3.

Current gap: profile's subparser has both --proxy-hints and --proxy-hints-with PATH=COLUMN, letting a user test a chi-squared proxy association against a protected column that was already dropped from the dataset. compare's subparser only registers --proxy-hints - there is no --proxy-hints-with argument at all in the compare parser block. SPEC.md states this explicitly: "Currently profile-only; compare's --proxy-hints does not accept --proxy-hints-with." proxy.py's parse_held_out_specs is already generic (takes any df), so the plumbing is already shared/reusable - only the CLI wiring for compare and calling proxy_hints(df_a/df_b, ..., held_out=...) twice is missing.

Repro:

$ grep -n "proxy-hints-with" faircode/cli.py
156:    p.add_argument("--proxy-hints-with", ...)   # only in the `profile` subparser block
$ grep -n "Currently .profile.-only" faircode/SPEC.md
134:Currently `profile`-only; `compare`'s `--proxy-hints` does not accept `--proxy-hints-with`.

Suggested approach: add --proxy-hints-with (repeatable, action="append") to the compare subparser, requiring the same PATH=COLUMN rows to align 1:1 with both csv_a and csv_b (or accept two separate flags, --proxy-hints-with-a/--proxy-hints-with-b, since A and B may have different row counts) and call proxy_hints(df_a, ..., held_out=held_out_a) / proxy_hints(df_b, ..., held_out=held_out_b) accordingly, reusing _build_held_out from cli.py.

[yakew7]

Fixed in 3815cf3: added --proxy-hints-with-a/--proxy-hints-with-b to faircode compare (previously profile-only), wired through faircode/proxy.py's existing held_out parameter via _build_held_out() for both datasets independently, with the same PATH=COLUMN parse/validation faircode profile already had.