feat(pushdown): generalise orderBy rendering beyond OData (#120); $select superset guard verified (#116)

[jeffreyaven]

Closes #120. Closes #116.

#120 - generalise orderBy pushdown beyond OData

applyPushdownOrderBy hard-required syntax: odata and always rendered
<column> <asc|desc>, so an API that expresses ordering any other way (OpenAI's
direction-only ?order=desc, GitHub's ?direction=asc, sort=-name prefix
styles) could not use the pushdown at all - unlike top/skip/count, which
honour any paramName.

orderBy already carries syntax as its render directive, and the docs listed
prefix and suffix values the code never rendered. The issue sketched a new
algorithm key; to avoid two overlapping knobs the new renderings are
additional syntax values:

syntax	renders	example
odata (default)	col asc|desc, comma-joined	$orderby=created_at desc
prefix	-col for desc, col for asc	sort=-created_at,name
suffix	col:asc|desc	sort=created_at:desc
column_only	column names only	sort=created_at
direction_only	asc/desc alone; exactly one term on an explicit supportedColumns allowlist	order=desc

Anything the syntax cannot express (unsupported column, more than one term for
direction_only, unknown syntax) emits nothing and ORDER BY stays client-side,
which remains authoritative. The odata path is byte-identical. No registry
provider currently declares an orderBy pushdown, so rendering prefix/suffix
changes no existing wire behaviour.

#116 - $select superset guard

Delivered in #124 (83c0a2e, released in v0.5.4-alpha01): applyPushdownSelect
extends the emitted $select with every column pushed into $filter or
$orderby, subject to the select allowlist, and ApplyPushdown runs filter and
order-by before select for that reason. The four superset tests from #124 cover
the issue's acceptance criteria (superset, unsupported column skipped, no change
when already projected, SELECT * still emits no $select) and pass here. No
further code is required; listed so the issue closes with the PR.

Verification

• go build ./..., gofmt, go test ./... clean; lint clean on changed files.
• New tests: direction_only asc/desc, refused for an unsupported column,
  multiple terms, or a missing allowlist; prefix/suffix/column_only
  rendering; unknown syntax emits nothing; OData default unchanged.
• End-to-end (stackql built against this branch vs v0.5.4-alpha01, local file
  registry, mock API logging raw queries), resource with
  orderBy: {paramName: order, syntax: direction_only, supportedColumns: [created_at]}:
  • ORDER BY created_at DESC -> /v1/containers?order=desc (baseline: no param)
  • ORDER BY created_at ASC -> /v1/containers?order=asc
  • ORDER BY id DESC -> no param, rows still ordered client-side

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