Thin isolation layer over PhoenixKitComments for every warehouse document kind.

Consolidates what were 5 near-byte-identical *_comments.ex wrapper modules in Andi (goods_issue_comments.ex, goods_receipt_comments.ex, internal_order_comments.ex, supplier_order_comments.ex, inventory_comments.ex) into one module parameterized by a kind atom. Every function returns a neutral value rather than raising when comments are unavailable, so callers never special-case it.

What "unavailable" does and does not cover

phoenix_kit_comments is a hard dependency (mix.exs, not optional:), so in practice "unavailable" means installed but disabled — the PhoenixKitComments.enabled?/0 half of available?/0. The Code.ensure_loaded?/1 half, like the @compile {:no_warn_undefined, ...} below, is always-true residue from treating the package as optional; both are harmless and left in place, but neither is what makes this module safe.

It does not cover version skew. available?/0 answers "is the module there and switched on", not "is it new enough": against comments 0.2.6 or 0.2.7 the module is present and enabled?/0 exists, so available?/0 returns true and subscribe/2, unsubscribe/2 or the list clause of count_comments/2 — all three added in 0.2.8 — would raise. That gap is closed by the >= 0.2.8 floor in mix.exs, not by any guard here.

:transfer was added later (Plan 4/T15) — transfers have no Andi predecessor, but plug into the same kind-parameterized wrapper.