fix: fit measured output reserves

[Brian Krabach (bkrabach)]

What changed

Add an optional fit_output(view, attempt) callback to the measured request-view path. After the existing eight legal reduction rungs still leave a native-measured request above its hard allowance, the request owner may return a newly counted output-reserve candidate. Context validates the exact counted envelope, reports reduced_output, and preserves staged commit/rollback behavior.

Why

Some provider budgets fit when the output reserve is reduced, without dropping required input. This provides that bounded negotiation while retaining existing protection and compatibility behavior.

Verification

• 219 tests passed, 1 existing xfail
• Ruff 0.14.10 check passed
• DTU validation: 208 passed, 11 abstract Core contract skips, 1 existing xfail
• Four-way old/new interoperability and rollback/protection behavior validated offline

Breaking changes

None. The callback is optional; callers without it retain the existing behavior. No defaults, core types, or required/tool protection rules changed.

The companion Loop change will be published separately after this dependency lands.