feat: add genLocal configuration option for grind

[leodemoura]

This PR adds a new grind configuration option, genLocal, that controls the
maximum term generation for local theorems (e.g., hypotheses). It defaults to
8, same value as gen and applies whenever
grind instantiates a theorem whose origin is local rather than a declaration
or user-provided term. Since users have little control over the patterns used
for local theorems, a tighter generation bound is a reasonable default.

Internally, E-matching now selects the generation bound via a new
getMaxGeneration helper that dispatches on the theorem's Origin: .stx and
.decl use gen, everything else uses genLocal.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 2674a1307cb6d4ad41915d852725534718b2b60b --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force Mathlib CI using the force-mathlib-ci label. (2026-05-11 03:14:10)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 2674a1307cb6d4ad41915d852725534718b2b60b --onto a71f158f7bd96ff9ea846f7ff4cc658de3c8b0f9. You can force reference manual CI using the force-manual-ci label. (2026-05-11 03:14:11)