This PR adds `at` location syntax to the `cbv` tactic, matching the
interface of `simp at`. Previously `cbv` could only reduce the goal
target; now it supports `cbv at h`, `cbv at h |-`, and `cbv at *`.
`cbvGoal` is rewritten to use `Sym.preprocessMVar` followed by `cbvCore`
within a single `SymM` context, sharing the term table across all
hypotheses and the target. The old `cbvGoalCore` (which reduced one side
of an equation goal at a time) is replaced by a general approach that
reduces arbitrary goal types and hypothesis types, with special handling
for `True` targets and `False` hypotheses. `cbvDecideGoal` is updated to
use the extracted `cbvCore` as well.
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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