--definitions-file can delete a semantic model or metric

[spascarella]

Problem

--definitions-file can create and evolve a semantic model or metric, but not remove one. Deleting means sending the whole file back without the definition, which is the cost the per-definition unit exists to remove.

The design question was what a dangling metric reference should do at plan time. The whole-plan delete guard reasons about deleted files and ref(); a definition removal deletes no file, and the name it takes away lives in a file that survives, so that guard cannot see it.

What changes

A definitions entry can carry "op": "delete", naming the definition and no content:

{"definitions": [{"kind": "metric", "name": "doubled", "op": "delete"}]}
Removal is always declared. Nothing is removed for going unmentioned, and one payload names each definition once, so the result cannot depend on read order.
semantic define refuses a removal; update and plan accept one, reported as a fourth class, removed.
A removal the surviving project still reads is refused, naming the reader and its file. Adding that reader's own delete or update to the same payload satisfies it.
A removal that would empty a file of semantic content is refused, pointing at transform plan --edits-file: deleting a file is a file-level act.
Comments and every other byte survive. A block's key: line goes only with its last item. DexEngine takes the same payload through definitions=.

Fix

Refuse rather than warn, because dbt would refuse anyway: a dangling metric input fails dbt parse, the gate every semantic plan already passes, so warning would only matter where that gate is off (--no-parse, no dbt) and would store a plan dex knows cannot be applied. maintain semantic stays the tool for drift that arrived on its own, not for a break this command is authoring.

check_removals repeats the delete guard's shape one namespace over: the project the payload leaves behind is built in memory and every surviving metric re-resolved against it, so the check reads the end state rather than the sequence. A removed semantic model takes its measures (create_metric included); a name re-declared elsewhere in the payload orphans nothing. Filter-string Metric() calls stay dbt's parser's to catch.

Known gap for its own issue: deleting a whole semantic YAML through transform plan still bypasses this guard, since validate_deletions reasons about ref().

Related issue

Closes #254