lint --fix: full-block-span removal for block-spanning Tier A fixes (empty-block, unreachable-branch)

[dean0x]

Problem

Current --fix planner (crates/mds-core/src/lint/fix.rs::diag_to_edit) removes only the directive line; for block-spanning rules this orphans the @end, so the reverify gate refuses every such fix (fail-closed — correct safety behavior, no user harm).

Solution

To make Tier A real for these rules, the planner needs a block-span edit:

• Compute the full span: directive-line start through the matching @end line (inclusive)
• Remove the entire block
• Add tests: fixed file must build byte-identically

Documented as best-effort in spec §8 footnote.

Discovered

Scenario QA on #61 / PR #171.

Labels

enhancement, cli (if those labels exist on the repo)